Vigilancia Tecnológica
Información de la temática del proyectoEpoxidized technical Kraft lignin as a particulate resin component for high-performance anticorrosive coatings
AbstractDeterioration of steel infrastructures is often caused by corrosive substances. In harsh conditions, the protection against corrosion is provided by high-performance coatings. The major challenge in this field is to find replacements for the fossil-based resins constituting anticorrosive coatings, due to increasing needs to synthesize new environmentally friendly materials. In this study, softwood Kraft lignin was epoxidized with the aim of obtaining a renewable resin for anticorrosive coatings. The reaction resulted in the formation of heterogeneous, solid, coarse agglomerates. Therefore, the synthetized lignin particles were mechanically ground and sieved to break up the agglomerates and obtain a fine powder. To reduce the use of fossil fuel-based epoxy novolac resins in commercial anticorrosive coatings, a series of formulations were prepared and cured on steel panels varying the content of epoxidized lignin resin. Epoxidized lignin-based coatings used in conjunction with conventional epoxy novolac resin demonstrated improved performance in terms of corrosion protection and adhesion properties, as measured by salt spray exposure and pull-off adhesion test, respectively. In addition, the importance of size fractionation for the homogeneity of the final coating formulations was highlighted. The findings from this study suggest a promising route to develop high-performing lignin-based anticorrosive coatings.
Similar content being viewed by others Preparation, development, outcomes, and application versatility of carbon fiber-based polymer composites: a review Article 08 January 2019 Tushar Kanti Das, Prosenjit Ghosh & Narayan Ch. Das
Decreasing the environmental impact of carbon fibre production via microwave carbonisation enabled by self-assembled nanostructured coatings Article Open access 23 February 2024 Micha? A. Stró?yk, Muhammad Muddasar, … Maurice N. Collins
A review of high-quality epoxy resins for corrosion-resistant applications Article 09 January 2024 Shams Anwar & Xianguo Li
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IntroductionThe term corrosion refers to the degradation of metals that occurs when they react with substances from the environment such as moisture and oxygen. This phenomenon is visible through the formation of rust and, if not mitigated, affects the performance of metals used in engineering and infrastructures. As a result, the corrosion protection market is one of the most important production sectors today. Several reports, including one from Global Market Insights,1 have shown that the market value of corrosion protection reached $13.5 billion in 2020 and will increase sharply in the coming years, with a forecast of $20 billion by 2027. Organic coatings represent the most commonly used method for corrosion protection of metallic materials and generally provide a protective barrier between the substrate and the environment with high resistance to ionic movement.2
Epoxy coatings account for most of the global value generated by the production of organic coatings. They are used to protect against corrosion in aggressive environments in equipment such as that used in oil and gas installations,3,4 and marine5 and energy applications.6 The base ingredients of epoxy coatings are epoxy resins, which combine excellent adhesion properties to metals with high resistance to harsh conditions such as heat, water, and certain chemicals.7 An epoxy resin and a curing agent, usually an amine compound, are mixed and a crosslinking reaction occurs, resulting in the formation of a thermoset polymer. Unparalleled corrosion protection is provided by the highly crosslinked molecular structure, which usually contains aromatic molecules. Good adhesion to substrates is also endured by the presence of pendant hydroxyl groups in the molecular structure.8 Considering all these properties, epoxy resins are often used as primers or intermediate coatings, on top of which a topcoat is usually applied.9 Most commercial epoxy coatings consist of bisphenol A (BPA) derivatives. The most commonly used for industrial applications is the diglycidyl ether bisphenol A (DGEBA). The aromatic segments of this molecule give the coatings high stiffness and good adhesion to the metal substrates.10 (BPA)-type epoxy coatings are usually combined with curing agents to form a binder formulation with high viscosity, which sometimes leads to difficulties in mixing.11 Therefore, alternatives such as bisphenol F (BPF)-type epoxy resins and phenol formaldehyde (n-PF) are commercially available. BPF-type epoxy resins have equivalent properties to the BPA types and also have lower viscosity, higher reactivity, and easy curing at ambient conditions.11 The n-PF resins, known commercially as epoxy novolacs, are used either alone or in combination with DGEBA.12 Apart from being based on fossil raw materials, environmental and human health concerns have been raised for materials made from bisphenol molecules.13,14,15
In order to have more sustainable corrosion protection systems, alternatives to the current fossil-based phenolic resins must be found while maintaining good coating performance. The most abundant natural resource for aromatic structures is lignin, a natural polymer that makes up 20% to 40% of wood tissue, depending on the species. Technical lignins are recovered as a side stream from pulping processes and are typically used for energy recovery. The amount of lignin produced worldwide is estimated at 50 to 70 million tons/year,16 but less is produced as refined lignin fractions. In recent years, new methods for separating lignin from the pulping process have been developed and used on a larger scale.17 It is therefore foreseeable that large quantities will be available in the near future.
Technical lignins are highly functional polymers containing phenolic, aliphatic, and carboxylic hydroxyl groups that can be chemically modified toward materials applications.18 Technical lignins differ significantly from native lignin. Their structure is highly dependent on the source and the method of separation. For example, technical Kraft lignin has a higher phenolic content than native lignin. The high functionality of technical lignins makes it an excellent candidate for chemical modifications to develop new bio-based materials.18 Technical lignins are mainly produced by a Kraft pulping process using sodium hydroxide (NaOH) and sodium sulfide (Na2S) to obtain black liquor as a product stream. Various processes can be used to separate lignin from the black liquor. One example is the LignoBoostTM process,17 which is based on the precipitation of lignin by changing the pH. Industrially, the separation of lignin from the Kraft process is rapidly growing compared to other extraction methods, such as lignosulfonates or organosolv.19
Several reaction pathways have been explored with the aim of exploiting the hydroxyl functionalities of Kraft lignin, such as acetylation,20 allylation,21,22 and silylation.23 Epoxidation of Kraft lignin has been investigated in previous studies after lignin fractionation24 or depolymerization,25 to obtain a more homogeneous system. This reaction is conventionally carried out by reactions of lignin hydroxyl groups with epichlorohydrin under alkaline conditions.26 Although functionalization of fractionated or depolymerized lignin has shown good results, scaling up to larger amounts still remains a challenge. To improve scalability, other epoxidation procedures have been carried out using technical Kraft27 or Organosolv lignin28 as retrieved, leading to promising results. In particular, the Organosolv lignin epoxidation approach developed by Over and co-workers28 has shown potentially scalable results. They reported a system containing DGEBA and 42 wt% epoxidized lignin showing higher glass transition temperature and better stiffness compared to a thermoset consisting of only DGEBA as resin.
Although the direct use of lignin is sometimes limited due to its high heterogeneity and polydispersity, which make it difficult to combine with conventional systems,29 a few studies have reported the incorporation of unmodified technical lignin in epoxy coatings as a substitute for pigments and fillers,30,31,32 showing good physical and mechanical properties. Laxminarayan et al.33 demonstrated a way to form homogeneous films by incorporating size-fractionated Kraft lignin particles, which were reduced to a suitable powder by mechanical milling and sieving. In this case, the introduction of unmodified lignin showed a significant improvement in adhesion performance compared to commercial coatings, as well as comparable long-term corrosion protection, after exposure for 70 days in a corrosive environment.
While the introduction of unmodified lignin has demonstrated the efficacy of a bio-based functional additive, the development of a scalable process to use epoxidized lignin as a resin component that is chemically crosslinked with curing agents is still an open research topic. In addition, there are no reliable tests in the literature showing the long-term corrosion resistance of coatings with incorporated lignin after epoxidation, and the results obtained are sometimes controversial. Previous electrochemical experiments conducted by Singh et al.34 with resins containing up to 20 wt% epoxidized lignin have shown poorer corrosion protection activity, while Komartin et al.35 have demonstrated promising performance in composites containing epoxidized linseed oil and Kraft lignin, resulting in a 140-380 % increase in corrosion protection.
The goal of the present work is twofold. One is to demonstrate a scalable route for the synthesis of epoxidized Kraft lignin (EKL) resins without further purification or fractionation processes; the other is to evaluate the properties of amine-cured epoxy novolacs when different amounts of EKL are used as resin component. Since epoxidized lignin particles are present in the coating matrix in particulate form, they could serve both as a resin and as substitute for pigments and additives. The present study demonstrates the possibility of scaling up a technique that would allow the use of epoxidized lignin as a bio-based component in engineering applications that typically require superior adhesion, corrosion, and mechanical resistance.
ExperimentalMaterialsEpoxidation of Kraft lignin was performed using the following compounds: softwood LignoBoostTM Kraft lignin (KL) kindly donated by StoraEnso (Finland); epichlorohydrin (ECH,???99%), potassium hydroxide (KOH,???90%), tetra-n-butylammonium bromide (TBAB,???98%), and n-hexane (? 99%) provided by Sigma-Aldrich; dichloromethane (DCM,???99.8%) provided by Supelco. Coatings formulations were prepared using the following chemicals: epoxy novolac resin DEN 438-X80 (EN); EPON 862 bisphenol F resin (DGEBF); pigment wet dispersant (WD) TroysperseTM CD1. The curing agent meta-xylenediamine 7% adduct (MXDA) was prepared by mixing meta-xylenediamine and bisphenol A (BPA) provided by Merck Life Science Aps (Denmark) and YTD-128 KUKDO Chemical Co. Ltd. (Korea), respectively. The curing agent cycloaliphatic amine hardener was supplied by Amicure PACM, Evonik Industries (Germany). All the other chemicals with analytical grade were received from Sigma-Aldrich.
Characterization analyses, particle modification, and performance evaluationPhosphorus nuclear magnetic resonance spectroscopy (31P NMR)Quantitative 31P NMR spectra were recorded at room temperature with a Bruker Advance III HD 400 MHz instrument with a BBFO probe and equipped with Z-gradient coil. All the data were analyzed by using MestReNova software V 14.2. This technique was utilized to obtain quantification of the different hydroxyl groups in the KL present before and after epoxidation, following the protocol developed by Argyropoulos in 1994.36 Accordingly, an internal standard (IS) solution was prepared by adding in 500 ?L of pyridine, ?30 mg N-hydroxy-5-norbornene?2,3-dicarboiamide (NHND, 97%), and ?3.5 mg of chromium(III) acetylacetonate used as relaxation agent. Thereafter, accurately 30 mg of lignin was dissolved in a solution containing 100 ?L of pyridine and 100 ?L of dimethylformamide (DMF). After complete dissolution, 50 ?L of IS and 100 µL of 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP, 95%) were added. TMDP was used as reagent able to phosphorylate the hydroxyl functionalities of lignin, subsequently forming a precipitate. The whole solution was redissolved in 400 ?L of deuterated chloroform (CDCl3). The spectra were acquired with an inverse-gated decoupling pulse (zgig30) with an acquisition time of 2 s, a relaxation delay of 10 s, and 128 number of scans. 1H NMR allowed the identification of epoxy functionalities. Then, ?15 mg of lignin samples was dissolved in 600 µL of deuterated DMSO (DMSO-d6).
Attenuated total reflection–Fourier transform infrared spectroscopy (ATR–FTIR)To analyze the degree of epoxidation of lignin specimens, FTIR analyses were performed using PerkinElmer 100 instrument in attenuated total reflection (ATR) mode with an MKII Golden Gate accessory (Specac Ltd.) equipped with a diamond crystal. FTIR analysis was also conducted to investigate the chemical structure of lignin-based epoxy coatings using a ThermoFisher Scientific Nicolet iS5 in ATR mode. Cured coatings were analyzed either directly as formed or after sanding of the surface. Measurements were obtained between 4000 and 600 cm?1, with a resolution of 4 cm?1 and 16 number of scans.
Differential scanning calorimetry (DSC)DSC was performed to analyze the thermal transitions of the samples. The first heating cycle was conducted from room temperature up to 100 °C and then isothermally kept for 5 min. Subsequently, the samples were cooled to ??20 °C and heated to 400 °C. During the second heating, glass transition (Tg) was detected.
Dynamic mechanical analysis (DMA)To analyze the viscoelastic properties, the coatings were cured as free films in silicon molds with a dimension of approximately 33 mm × 5.5 mm × 1.5 mm and clamped in a DMA Q800 equipped with pressurized air-cooling system. The samples were subjected to a strain of 0.1%, preload force of 0.1 N, and a force track of 125.0%. The samples were initially cooled down at 0 °C followed by isothermal for 5 min and then a ramping temperature of 3 °C/min to 180 °C. Sample specimens were either tested directly after ambient curing or after being post-cured at 100 °C in an oven for 2 h.
Scanning electron microscopy (SEM)The observation of the cross section of lignin-based films was conducted on a field emission Hitachi SEM S-4800 microscope in order to investigate the internal structure of the materials. The images of the specimens were observed on a field emission Hitachi SEM S-4800 microscope. The images were captured at different magnifications (from 500 to 10000) and accelerating voltage (1.0, 1.2, or 1.6 kV) at a working distance of 8.5–9.5 mm. The cross sections were prepared by breaking the free films specimens using a scalpel. The cross sections of samples were placed on conductive carbon tape. All samples were sputtered with Pt/Pd system for 30 s, at a current of 80 mA with a Cressington 208HR sputter coater. The coating layer formed was about 2 nm thick, measured through quartz crystal microbalance.
Size fractionation of ligninA Planetary Ball Mill PM 100 by Retsch was utilized in order to perform the grinding procedure of epoxidized lignin resin particles. The equipment consisted in a compact bench top single grinding station. Three steel balls mill (Ø30 mm) and approximately 25 g of lignin particles were put in the grinding jar which was successively clumped into the grinding station. Then, the milling procedure was started at 400 rpm for 10 min and repeated three times, obtaining a light brown fine powder. After grinding the particles, an analytical sieve shaker Retsch AS 200 control was used to sieve the powder to give a fine resin using space meshes of 63 µm. The sieving time was set at 15 min with an amplitude of 1.70 mm/gravity. After sieving, the particles with larger size than the meshes were collected and the grinding and sieving procedure was repeated to increase the overall yield until 95 wt%.
Laser diffraction particle size analysisAccording to Mie scattering theory (ISO 13320-137 and ISO 9276-138), the size distribution of epoxidized lignin particles before and after size fractionation was measured using the laser diffraction MasterSizer 3000 from Malvern Technology Company. The particle and dispersant refractive index were defined, respectively, as 1.61 and 1.0. Ten repetitions for each sample were conducted in order to obtain an accurate standard deviation.
Pull-off adhesionTo test the adhesive resistance of coated surfaces, the pull-off test was performed according to EN ISO 4624:2016,39 utilizing an Elcometer model 510. Two aluminum dollies (Ø10 mm) were applied on the surface of the coatings using a formulated two-component BPF epoxy-amine adduct adhesive glue (similar to Araldite) and allowed to dry for more than 24 h. The surfaces were sanded prior to gluing of the dollies in order to increase the contact area. The dollies were pulled at a pull rate of 0.7 MPa/s, and the final force required to break the coating bond to the panel was measured.
Salt spray exposureSalt spray test was performed to evaluate long-term corrosion resistance and to detect defects in coatings, according to EN ISO 9227-2022.40 A scribe of 50 mm × 2 mm was created in the cured film down to the steel substrate, using CNC milling machine. Then, according to the standard procedure, all the sides and the back of the substrates were coated with a commercial coating (Hempaline Defend 630). The tests were carried out in an enclosed, temperature-controlled tank where the samples were placed in specific supports. A 5 wt% sodium chloride solution was atomized from the nozzles, and the chamber was sealed and maintained at 35?±?2 °C with a humidity level of 50?±?1% for the entire duration of the tests.
Rust creep assessmentThe rust creep was measured after 70 days of salt spray exposure, evaluating the thickness of delamination around the scribe of the coatings previously created. The coating around the scribe was removed by positioning a knife blade between the film and the substrate and applying a certain force manually. According to EN ISO 12944-9:2018,41 the rust creep value, M, was evaluated using the following equation
$$M=frac{C-W}{2}$$where C is the average of nine width measurements and W is the width of the scribe.
Pendulum hardnessHardness of coatings was evaluated according to ISO 152242 by measuring the damping time of a König oscillating pendulum model Erichsen 299/300. The coated glass panels (DFT?=?70?±?5 µm) of dimension 100 mm × 150 mm × 6 mm were placed under the pendulum initially deflected to 6° and released, while a digital oscillation counter started at the same time. The number of oscillations of the pendulum was counted from 6° until it reached 3°. The number of oscillations was converted in seconds by multiplying the number of oscillations for 1.4 s.42
Direct impact resistanceThe resistance, deformability, and ductility of the coatings to impacts were measured through impact tester Erichsen model 304. According to ISO 6272-1:2011,43 a ball (Ø20 mm/0.79?) of 1 kg was dropped freely down a guide tube with increasing height, until cracks were observed on the surface. The deformation of the surface after direct impacts was investigated by a digital microscope Keyence VHX-6000. The images were captured between 20.0x and 30.0x of magnification.
Synthesis and coatings preparationKraft lignin epoxidationRaw KL (50 g–336 mmol OH) was suspended in excess of epichlorohydrin (ECH) (100 ml–1.28 mol) in a three-neck round-bottom flask equipped with a condenser and immersed in an ice bath. Then, TBAB (9 g–59.2 mmol) was added in portions and the mixture was blended for 1 h. KOH in the form of flakes (1.59 eq. the number of OH functionalities of lignin) was slowly added during vigorous mixing, over the course of 2.5 h. The reaction mixture was then stirred at room temperature for 5 h. The mixture was then diluted with DCM (300 ml) and the organic phase extracted with water (5 ml × 200 ml). The organic phase was then concentrated to roughly 100 ml by evaporating solvents with a rotary evaporator. Finally, the product was precipitated in n-hexane, decanted, and the residue dried in a vacuum oven for 24 h at 50 °C resulting in a hard-brown cake. The product (EKL) was manually ground in a mortar, redispersed in water, and agitated overnight to produce a coarse particle dispersion, which was then filtered and dried in a vacuum oven for additional 24 h. The amount of EKL obtained ranged between 40 and 55 g depending on losses during the experiment. EKL particles were then mechanically ground and sieved to size-fractionate the particles resin using a mesh size of 63 µm. Figure 1 depicts the difference between size-fractionated epoxidized Kraft lignin (SF-EKL) and nonsize-fractionated epoxidized Kraft lignin (NF-EKL). It is proposed that the SF-EKL appears less dark due to light-scattering effects.
Fig. 1Examples of NF-EKL (left) and SF-EKL (right) resins
Full size imageCoatings preparationThe base components of coatings formulation, i.e., epoxy novolac/DGEBF resin, epoxidized lignin (S-EKL/N-EKL), and wetting–dispersing agent, were stirred using a high-speed disperser (DISPERMAT CV3-PLUS, VMA-Getzmann GmbH, Germany) at approximately 5000 rpm until a well-dispersed formulation was formed. The epoxidized lignin resin replaced exactly 0, 25, or 50 wt% of the traditional resin depending on formulation. Xylene was then added dropwise to achieve the ideal coating viscosity. Finally, the amine curing agent (MXDA-adduct for EN-based or Amicure for DGEBF-based coatings) was added to the formulations in the amounts indicated by the stoichiometric ratios (SR) in Table 1. The final coating formulations were applied to grit-blasted mild steel panels (75 mm?×?150 mm?×?3 mm) that had been industrially sandblasted to a cleanliness of Sa 2½ in accordance with ISO 8503-144 with a medium (G) surface roughness profile in accordance with ISO 8501-1.45 A steel bar applicator was used to apply all coats. The coated panels were cured at room temperature for 7 days. After curing, the dry film thickness (DFT) was 300?±?10 µm, accurately measured using a coating thickness gauge (Elcometer 355). The list of coating formulations is presented in Table 1. In Table 2 are schematized the molecular structures of the components utilized in coating formulations.
Table 1 Summary of the coating formulations prepared. SF-EKL?=?size-fractionated epoxy Kraft lignin, NF-EKL?=?nonfractionated epoxy Kraft lignin, EN?=?epoxy novolac, DGEBF?=?bisphenol F-diglycidyl ether. Coatings (a), (b), and (c) are all clear coatings using conventional resinsFull size tableTable 2 Molecular structure of the materials used for the different coating formulationsFull size tableResults and discussionResin synthesisEpoxidation of KL with epichlorohydrin utilizing TBAB and KOH catalysts was initiated in an ice bath (Scheme 1). The presence of the ice bath at the beginning of the reaction was required to maintain the temperature of this exothermic process below 50 °C, in order to avoid risk of side reactions.28,46,47
Scheme 1Representation of the Kraft lignin epoxidation reaction
Full size imageThe EEW of EKL was estimated by comparison of the 31P NMR before and after epoxidation, as well as considering the additional molecular weight of the epoxy groups (Fig. 2). All of the phenolic and carboxylic hydroxyls reacted completely. Some aliphatic alcohol groups remained unreacted, and a new signal was detected between 145 and 146.5 ppm. The presence of the novel signal was previously explored and determined to be the result of either NMR-phosphite derivatization of the phenolic-linked epoxide or from incomplete synthesis of the oxirane ring after epoxidation.24
Fig. 231P NMR spectra of unmodified Kraft lignin (top) and epoxidized Kraft lignin (bottom)
Full size imageIndeed, the phenolic and carboxylic acid hydroxyl groups totally reacted, decreasing from 3.8 and 0.4 to 0 mmol g?1, respectively. Because phenolic and carboxylic acid hydroxyls are known to react with epoxides, removing these groups completely ensures that the EKL can only chemically react with the amine crosslinker. If the remaining acid or phenolic functionalities are not eliminated, they may influence the stoichiometric ratio of epoxides and amines, making the data more difficult to interpret.
A summary of the quantification of functional groups before and after epoxidation is shown in Table 3. The value of EEW of EKL was calculated according to the results obtained from the evaluation of 31P NMR.
Table 3 Estimation of epoxy content after functionalization by 31P NMR quantificationFull size tableComparison of the FTIR spectra in Fig. 3 showed more evidence of lignin epoxidation. Stretching vibrations of hydroxyls functionalities between 3700 and 3100 cm?1 are greatly reduced after epoxidation, validating previous findings from 31P NMR spectra. The epoxidation additionally increases the vibration signal of the C–H bonding vibration signal between 3100 and 2750 cm?1. The existence of C–O–glycidyl (ether) is shown at 1089 cm?1, and the new signals at 908 and 755 cm?1 are indicative of C–O–C oxirane asymmetric stretching vibration. These findings are consistent with previous studies on lignin epoxidation.25,28,48,49 According to the FTIR data, the new peak in the 31P NMR spectrum resulted from an interaction between an epoxide and the phosphorylating agent.
Fig. 3FTIR spectra of unmodified Kraft lignin (top) and epoxidized Kraft lignin (bottom)
Full size imageThe particle size distribution was determined using a laser diffraction particle size analyzer. As indicated in Fig. 4, the particle size of the sieved and ground particles was less that of the nonsieved. The average particle size of SF-EKL was 51 µm, while that of NF-EKL was 139 µm. When assuming a smooth spherical particle shape, the specific surface area of NF-EKL was determined to be 74.3 m2 kg?1. The surface area increased to 1148 m2 kg?1 after grinding and sieving operations. This result indicates that the contact area between the SF-EKL and the binders was 15 times higher than for NF-EKL, which contributed significantly to the coatings’ ultimate homogeneity and performance. Figure S16 depicts particle patterns and shapes as determined by SEM analyses.
Fig. 4Particle size distributions (10 repetitions) of SF-EKL (top) and NF-EKL (bottom) by laser diffraction particle size analysis
Full size imageCoatings formulations and film formationA high-speed disperser was used to create a variety of coating compositions. The epoxidized lignin was produced as a particulate solid resin that was xylene insoluble. As a result, the coating compositions were made up of a mix of conventional epoxy resins, hardener, xylene, and dispersed epoxidized lignin resin particles. Curing and drying at room temperature allowed for resemble procedures commonly employed in industry. This, however, raises the possibility of vitrification and post-curing consequences as shown in the thermal properties paragraph. Except for the coating containing NF-EKL, which had a rough agglomerated topography, all of the dried and cured films had a smooth surface, as illustrated in Fig. 6. According to scheme 2, the curing process was the result of a combination between physical drying by solvent evaporation and a chemical crosslinking.
Scheme 2
Similar content being viewed by others Preparation, development, outcomes, and application versatility of carbon fiber-based polymer composites: a review Article 08 January 2019 Tushar Kanti Das, Prosenjit Ghosh & Narayan Ch. Das
Decreasing the environmental impact of carbon fibre production via microwave carbonisation enabled by self-assembled nanostructured coatings Article Open access 23 February 2024 Micha? A. Stró?yk, Muhammad Muddasar, … Maurice N. Collins
A review of high-quality epoxy resins for corrosion-resistant applications Article 09 January 2024 Shams Anwar & Xianguo Li
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IntroductionThe term corrosion refers to the degradation of metals that occurs when they react with substances from the environment such as moisture and oxygen. This phenomenon is visible through the formation of rust and, if not mitigated, affects the performance of metals used in engineering and infrastructures. As a result, the corrosion protection market is one of the most important production sectors today. Several reports, including one from Global Market Insights,1 have shown that the market value of corrosion protection reached $13.5 billion in 2020 and will increase sharply in the coming years, with a forecast of $20 billion by 2027. Organic coatings represent the most commonly used method for corrosion protection of metallic materials and generally provide a protective barrier between the substrate and the environment with high resistance to ionic movement.2
Epoxy coatings account for most of the global value generated by the production of organic coatings. They are used to protect against corrosion in aggressive environments in equipment such as that used in oil and gas installations,3,4 and marine5 and energy applications.6 The base ingredients of epoxy coatings are epoxy resins, which combine excellent adhesion properties to metals with high resistance to harsh conditions such as heat, water, and certain chemicals.7 An epoxy resin and a curing agent, usually an amine compound, are mixed and a crosslinking reaction occurs, resulting in the formation of a thermoset polymer. Unparalleled corrosion protection is provided by the highly crosslinked molecular structure, which usually contains aromatic molecules. Good adhesion to substrates is also endured by the presence of pendant hydroxyl groups in the molecular structure.8 Considering all these properties, epoxy resins are often used as primers or intermediate coatings, on top of which a topcoat is usually applied.9 Most commercial epoxy coatings consist of bisphenol A (BPA) derivatives. The most commonly used for industrial applications is the diglycidyl ether bisphenol A (DGEBA). The aromatic segments of this molecule give the coatings high stiffness and good adhesion to the metal substrates.10 (BPA)-type epoxy coatings are usually combined with curing agents to form a binder formulation with high viscosity, which sometimes leads to difficulties in mixing.11 Therefore, alternatives such as bisphenol F (BPF)-type epoxy resins and phenol formaldehyde (n-PF) are commercially available. BPF-type epoxy resins have equivalent properties to the BPA types and also have lower viscosity, higher reactivity, and easy curing at ambient conditions.11 The n-PF resins, known commercially as epoxy novolacs, are used either alone or in combination with DGEBA.12 Apart from being based on fossil raw materials, environmental and human health concerns have been raised for materials made from bisphenol molecules.13,14,15
In order to have more sustainable corrosion protection systems, alternatives to the current fossil-based phenolic resins must be found while maintaining good coating performance. The most abundant natural resource for aromatic structures is lignin, a natural polymer that makes up 20% to 40% of wood tissue, depending on the species. Technical lignins are recovered as a side stream from pulping processes and are typically used for energy recovery. The amount of lignin produced worldwide is estimated at 50 to 70 million tons/year,16 but less is produced as refined lignin fractions. In recent years, new methods for separating lignin from the pulping process have been developed and used on a larger scale.17 It is therefore foreseeable that large quantities will be available in the near future.
Technical lignins are highly functional polymers containing phenolic, aliphatic, and carboxylic hydroxyl groups that can be chemically modified toward materials applications.18 Technical lignins differ significantly from native lignin. Their structure is highly dependent on the source and the method of separation. For example, technical Kraft lignin has a higher phenolic content than native lignin. The high functionality of technical lignins makes it an excellent candidate for chemical modifications to develop new bio-based materials.18 Technical lignins are mainly produced by a Kraft pulping process using sodium hydroxide (NaOH) and sodium sulfide (Na2S) to obtain black liquor as a product stream. Various processes can be used to separate lignin from the black liquor. One example is the LignoBoostTM process,17 which is based on the precipitation of lignin by changing the pH. Industrially, the separation of lignin from the Kraft process is rapidly growing compared to other extraction methods, such as lignosulfonates or organosolv.19
Several reaction pathways have been explored with the aim of exploiting the hydroxyl functionalities of Kraft lignin, such as acetylation,20 allylation,21,22 and silylation.23 Epoxidation of Kraft lignin has been investigated in previous studies after lignin fractionation24 or depolymerization,25 to obtain a more homogeneous system. This reaction is conventionally carried out by reactions of lignin hydroxyl groups with epichlorohydrin under alkaline conditions.26 Although functionalization of fractionated or depolymerized lignin has shown good results, scaling up to larger amounts still remains a challenge. To improve scalability, other epoxidation procedures have been carried out using technical Kraft27 or Organosolv lignin28 as retrieved, leading to promising results. In particular, the Organosolv lignin epoxidation approach developed by Over and co-workers28 has shown potentially scalable results. They reported a system containing DGEBA and 42 wt% epoxidized lignin showing higher glass transition temperature and better stiffness compared to a thermoset consisting of only DGEBA as resin.
Although the direct use of lignin is sometimes limited due to its high heterogeneity and polydispersity, which make it difficult to combine with conventional systems,29 a few studies have reported the incorporation of unmodified technical lignin in epoxy coatings as a substitute for pigments and fillers,30,31,32 showing good physical and mechanical properties. Laxminarayan et al.33 demonstrated a way to form homogeneous films by incorporating size-fractionated Kraft lignin particles, which were reduced to a suitable powder by mechanical milling and sieving. In this case, the introduction of unmodified lignin showed a significant improvement in adhesion performance compared to commercial coatings, as well as comparable long-term corrosion protection, after exposure for 70 days in a corrosive environment.
While the introduction of unmodified lignin has demonstrated the efficacy of a bio-based functional additive, the development of a scalable process to use epoxidized lignin as a resin component that is chemically crosslinked with curing agents is still an open research topic. In addition, there are no reliable tests in the literature showing the long-term corrosion resistance of coatings with incorporated lignin after epoxidation, and the results obtained are sometimes controversial. Previous electrochemical experiments conducted by Singh et al.34 with resins containing up to 20 wt% epoxidized lignin have shown poorer corrosion protection activity, while Komartin et al.35 have demonstrated promising performance in composites containing epoxidized linseed oil and Kraft lignin, resulting in a 140-380 % increase in corrosion protection.
The goal of the present work is twofold. One is to demonstrate a scalable route for the synthesis of epoxidized Kraft lignin (EKL) resins without further purification or fractionation processes; the other is to evaluate the properties of amine-cured epoxy novolacs when different amounts of EKL are used as resin component. Since epoxidized lignin particles are present in the coating matrix in particulate form, they could serve both as a resin and as substitute for pigments and additives. The present study demonstrates the possibility of scaling up a technique that would allow the use of epoxidized lignin as a bio-based component in engineering applications that typically require superior adhesion, corrosion, and mechanical resistance.
ExperimentalMaterialsEpoxidation of Kraft lignin was performed using the following compounds: softwood LignoBoostTM Kraft lignin (KL) kindly donated by StoraEnso (Finland); epichlorohydrin (ECH,???99%), potassium hydroxide (KOH,???90%), tetra-n-butylammonium bromide (TBAB,???98%), and n-hexane (? 99%) provided by Sigma-Aldrich; dichloromethane (DCM,???99.8%) provided by Supelco. Coatings formulations were prepared using the following chemicals: epoxy novolac resin DEN 438-X80 (EN); EPON 862 bisphenol F resin (DGEBF); pigment wet dispersant (WD) TroysperseTM CD1. The curing agent meta-xylenediamine 7% adduct (MXDA) was prepared by mixing meta-xylenediamine and bisphenol A (BPA) provided by Merck Life Science Aps (Denmark) and YTD-128 KUKDO Chemical Co. Ltd. (Korea), respectively. The curing agent cycloaliphatic amine hardener was supplied by Amicure PACM, Evonik Industries (Germany). All the other chemicals with analytical grade were received from Sigma-Aldrich.
Characterization analyses, particle modification, and performance evaluationPhosphorus nuclear magnetic resonance spectroscopy (31P NMR)Quantitative 31P NMR spectra were recorded at room temperature with a Bruker Advance III HD 400 MHz instrument with a BBFO probe and equipped with Z-gradient coil. All the data were analyzed by using MestReNova software V 14.2. This technique was utilized to obtain quantification of the different hydroxyl groups in the KL present before and after epoxidation, following the protocol developed by Argyropoulos in 1994.36 Accordingly, an internal standard (IS) solution was prepared by adding in 500 ?L of pyridine, ?30 mg N-hydroxy-5-norbornene?2,3-dicarboiamide (NHND, 97%), and ?3.5 mg of chromium(III) acetylacetonate used as relaxation agent. Thereafter, accurately 30 mg of lignin was dissolved in a solution containing 100 ?L of pyridine and 100 ?L of dimethylformamide (DMF). After complete dissolution, 50 ?L of IS and 100 µL of 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP, 95%) were added. TMDP was used as reagent able to phosphorylate the hydroxyl functionalities of lignin, subsequently forming a precipitate. The whole solution was redissolved in 400 ?L of deuterated chloroform (CDCl3). The spectra were acquired with an inverse-gated decoupling pulse (zgig30) with an acquisition time of 2 s, a relaxation delay of 10 s, and 128 number of scans. 1H NMR allowed the identification of epoxy functionalities. Then, ?15 mg of lignin samples was dissolved in 600 µL of deuterated DMSO (DMSO-d6).
Attenuated total reflection–Fourier transform infrared spectroscopy (ATR–FTIR)To analyze the degree of epoxidation of lignin specimens, FTIR analyses were performed using PerkinElmer 100 instrument in attenuated total reflection (ATR) mode with an MKII Golden Gate accessory (Specac Ltd.) equipped with a diamond crystal. FTIR analysis was also conducted to investigate the chemical structure of lignin-based epoxy coatings using a ThermoFisher Scientific Nicolet iS5 in ATR mode. Cured coatings were analyzed either directly as formed or after sanding of the surface. Measurements were obtained between 4000 and 600 cm?1, with a resolution of 4 cm?1 and 16 number of scans.
Differential scanning calorimetry (DSC)DSC was performed to analyze the thermal transitions of the samples. The first heating cycle was conducted from room temperature up to 100 °C and then isothermally kept for 5 min. Subsequently, the samples were cooled to ??20 °C and heated to 400 °C. During the second heating, glass transition (Tg) was detected.
Dynamic mechanical analysis (DMA)To analyze the viscoelastic properties, the coatings were cured as free films in silicon molds with a dimension of approximately 33 mm × 5.5 mm × 1.5 mm and clamped in a DMA Q800 equipped with pressurized air-cooling system. The samples were subjected to a strain of 0.1%, preload force of 0.1 N, and a force track of 125.0%. The samples were initially cooled down at 0 °C followed by isothermal for 5 min and then a ramping temperature of 3 °C/min to 180 °C. Sample specimens were either tested directly after ambient curing or after being post-cured at 100 °C in an oven for 2 h.
Scanning electron microscopy (SEM)The observation of the cross section of lignin-based films was conducted on a field emission Hitachi SEM S-4800 microscope in order to investigate the internal structure of the materials. The images of the specimens were observed on a field emission Hitachi SEM S-4800 microscope. The images were captured at different magnifications (from 500 to 10000) and accelerating voltage (1.0, 1.2, or 1.6 kV) at a working distance of 8.5–9.5 mm. The cross sections were prepared by breaking the free films specimens using a scalpel. The cross sections of samples were placed on conductive carbon tape. All samples were sputtered with Pt/Pd system for 30 s, at a current of 80 mA with a Cressington 208HR sputter coater. The coating layer formed was about 2 nm thick, measured through quartz crystal microbalance.
Size fractionation of ligninA Planetary Ball Mill PM 100 by Retsch was utilized in order to perform the grinding procedure of epoxidized lignin resin particles. The equipment consisted in a compact bench top single grinding station. Three steel balls mill (Ø30 mm) and approximately 25 g of lignin particles were put in the grinding jar which was successively clumped into the grinding station. Then, the milling procedure was started at 400 rpm for 10 min and repeated three times, obtaining a light brown fine powder. After grinding the particles, an analytical sieve shaker Retsch AS 200 control was used to sieve the powder to give a fine resin using space meshes of 63 µm. The sieving time was set at 15 min with an amplitude of 1.70 mm/gravity. After sieving, the particles with larger size than the meshes were collected and the grinding and sieving procedure was repeated to increase the overall yield until 95 wt%.
Laser diffraction particle size analysisAccording to Mie scattering theory (ISO 13320-137 and ISO 9276-138), the size distribution of epoxidized lignin particles before and after size fractionation was measured using the laser diffraction MasterSizer 3000 from Malvern Technology Company. The particle and dispersant refractive index were defined, respectively, as 1.61 and 1.0. Ten repetitions for each sample were conducted in order to obtain an accurate standard deviation.
Pull-off adhesionTo test the adhesive resistance of coated surfaces, the pull-off test was performed according to EN ISO 4624:2016,39 utilizing an Elcometer model 510. Two aluminum dollies (Ø10 mm) were applied on the surface of the coatings using a formulated two-component BPF epoxy-amine adduct adhesive glue (similar to Araldite) and allowed to dry for more than 24 h. The surfaces were sanded prior to gluing of the dollies in order to increase the contact area. The dollies were pulled at a pull rate of 0.7 MPa/s, and the final force required to break the coating bond to the panel was measured.
Salt spray exposureSalt spray test was performed to evaluate long-term corrosion resistance and to detect defects in coatings, according to EN ISO 9227-2022.40 A scribe of 50 mm × 2 mm was created in the cured film down to the steel substrate, using CNC milling machine. Then, according to the standard procedure, all the sides and the back of the substrates were coated with a commercial coating (Hempaline Defend 630). The tests were carried out in an enclosed, temperature-controlled tank where the samples were placed in specific supports. A 5 wt% sodium chloride solution was atomized from the nozzles, and the chamber was sealed and maintained at 35?±?2 °C with a humidity level of 50?±?1% for the entire duration of the tests.
Rust creep assessmentThe rust creep was measured after 70 days of salt spray exposure, evaluating the thickness of delamination around the scribe of the coatings previously created. The coating around the scribe was removed by positioning a knife blade between the film and the substrate and applying a certain force manually. According to EN ISO 12944-9:2018,41 the rust creep value, M, was evaluated using the following equation
$$M=frac{C-W}{2}$$where C is the average of nine width measurements and W is the width of the scribe.
Pendulum hardnessHardness of coatings was evaluated according to ISO 152242 by measuring the damping time of a König oscillating pendulum model Erichsen 299/300. The coated glass panels (DFT?=?70?±?5 µm) of dimension 100 mm × 150 mm × 6 mm were placed under the pendulum initially deflected to 6° and released, while a digital oscillation counter started at the same time. The number of oscillations of the pendulum was counted from 6° until it reached 3°. The number of oscillations was converted in seconds by multiplying the number of oscillations for 1.4 s.42
Direct impact resistanceThe resistance, deformability, and ductility of the coatings to impacts were measured through impact tester Erichsen model 304. According to ISO 6272-1:2011,43 a ball (Ø20 mm/0.79?) of 1 kg was dropped freely down a guide tube with increasing height, until cracks were observed on the surface. The deformation of the surface after direct impacts was investigated by a digital microscope Keyence VHX-6000. The images were captured between 20.0x and 30.0x of magnification.
Synthesis and coatings preparationKraft lignin epoxidationRaw KL (50 g–336 mmol OH) was suspended in excess of epichlorohydrin (ECH) (100 ml–1.28 mol) in a three-neck round-bottom flask equipped with a condenser and immersed in an ice bath. Then, TBAB (9 g–59.2 mmol) was added in portions and the mixture was blended for 1 h. KOH in the form of flakes (1.59 eq. the number of OH functionalities of lignin) was slowly added during vigorous mixing, over the course of 2.5 h. The reaction mixture was then stirred at room temperature for 5 h. The mixture was then diluted with DCM (300 ml) and the organic phase extracted with water (5 ml × 200 ml). The organic phase was then concentrated to roughly 100 ml by evaporating solvents with a rotary evaporator. Finally, the product was precipitated in n-hexane, decanted, and the residue dried in a vacuum oven for 24 h at 50 °C resulting in a hard-brown cake. The product (EKL) was manually ground in a mortar, redispersed in water, and agitated overnight to produce a coarse particle dispersion, which was then filtered and dried in a vacuum oven for additional 24 h. The amount of EKL obtained ranged between 40 and 55 g depending on losses during the experiment. EKL particles were then mechanically ground and sieved to size-fractionate the particles resin using a mesh size of 63 µm. Figure 1 depicts the difference between size-fractionated epoxidized Kraft lignin (SF-EKL) and nonsize-fractionated epoxidized Kraft lignin (NF-EKL). It is proposed that the SF-EKL appears less dark due to light-scattering effects.
Fig. 1Examples of NF-EKL (left) and SF-EKL (right) resins
Full size imageCoatings preparationThe base components of coatings formulation, i.e., epoxy novolac/DGEBF resin, epoxidized lignin (S-EKL/N-EKL), and wetting–dispersing agent, were stirred using a high-speed disperser (DISPERMAT CV3-PLUS, VMA-Getzmann GmbH, Germany) at approximately 5000 rpm until a well-dispersed formulation was formed. The epoxidized lignin resin replaced exactly 0, 25, or 50 wt% of the traditional resin depending on formulation. Xylene was then added dropwise to achieve the ideal coating viscosity. Finally, the amine curing agent (MXDA-adduct for EN-based or Amicure for DGEBF-based coatings) was added to the formulations in the amounts indicated by the stoichiometric ratios (SR) in Table 1. The final coating formulations were applied to grit-blasted mild steel panels (75 mm?×?150 mm?×?3 mm) that had been industrially sandblasted to a cleanliness of Sa 2½ in accordance with ISO 8503-144 with a medium (G) surface roughness profile in accordance with ISO 8501-1.45 A steel bar applicator was used to apply all coats. The coated panels were cured at room temperature for 7 days. After curing, the dry film thickness (DFT) was 300?±?10 µm, accurately measured using a coating thickness gauge (Elcometer 355). The list of coating formulations is presented in Table 1. In Table 2 are schematized the molecular structures of the components utilized in coating formulations.
Table 1 Summary of the coating formulations prepared. SF-EKL?=?size-fractionated epoxy Kraft lignin, NF-EKL?=?nonfractionated epoxy Kraft lignin, EN?=?epoxy novolac, DGEBF?=?bisphenol F-diglycidyl ether. Coatings (a), (b), and (c) are all clear coatings using conventional resinsFull size tableTable 2 Molecular structure of the materials used for the different coating formulationsFull size tableResults and discussionResin synthesisEpoxidation of KL with epichlorohydrin utilizing TBAB and KOH catalysts was initiated in an ice bath (Scheme 1). The presence of the ice bath at the beginning of the reaction was required to maintain the temperature of this exothermic process below 50 °C, in order to avoid risk of side reactions.28,46,47
Scheme 1Representation of the Kraft lignin epoxidation reaction
Full size imageThe EEW of EKL was estimated by comparison of the 31P NMR before and after epoxidation, as well as considering the additional molecular weight of the epoxy groups (Fig. 2). All of the phenolic and carboxylic hydroxyls reacted completely. Some aliphatic alcohol groups remained unreacted, and a new signal was detected between 145 and 146.5 ppm. The presence of the novel signal was previously explored and determined to be the result of either NMR-phosphite derivatization of the phenolic-linked epoxide or from incomplete synthesis of the oxirane ring after epoxidation.24
Fig. 231P NMR spectra of unmodified Kraft lignin (top) and epoxidized Kraft lignin (bottom)
Full size imageIndeed, the phenolic and carboxylic acid hydroxyl groups totally reacted, decreasing from 3.8 and 0.4 to 0 mmol g?1, respectively. Because phenolic and carboxylic acid hydroxyls are known to react with epoxides, removing these groups completely ensures that the EKL can only chemically react with the amine crosslinker. If the remaining acid or phenolic functionalities are not eliminated, they may influence the stoichiometric ratio of epoxides and amines, making the data more difficult to interpret.
A summary of the quantification of functional groups before and after epoxidation is shown in Table 3. The value of EEW of EKL was calculated according to the results obtained from the evaluation of 31P NMR.
Table 3 Estimation of epoxy content after functionalization by 31P NMR quantificationFull size tableComparison of the FTIR spectra in Fig. 3 showed more evidence of lignin epoxidation. Stretching vibrations of hydroxyls functionalities between 3700 and 3100 cm?1 are greatly reduced after epoxidation, validating previous findings from 31P NMR spectra. The epoxidation additionally increases the vibration signal of the C–H bonding vibration signal between 3100 and 2750 cm?1. The existence of C–O–glycidyl (ether) is shown at 1089 cm?1, and the new signals at 908 and 755 cm?1 are indicative of C–O–C oxirane asymmetric stretching vibration. These findings are consistent with previous studies on lignin epoxidation.25,28,48,49 According to the FTIR data, the new peak in the 31P NMR spectrum resulted from an interaction between an epoxide and the phosphorylating agent.
Fig. 3FTIR spectra of unmodified Kraft lignin (top) and epoxidized Kraft lignin (bottom)
Full size imageThe particle size distribution was determined using a laser diffraction particle size analyzer. As indicated in Fig. 4, the particle size of the sieved and ground particles was less that of the nonsieved. The average particle size of SF-EKL was 51 µm, while that of NF-EKL was 139 µm. When assuming a smooth spherical particle shape, the specific surface area of NF-EKL was determined to be 74.3 m2 kg?1. The surface area increased to 1148 m2 kg?1 after grinding and sieving operations. This result indicates that the contact area between the SF-EKL and the binders was 15 times higher than for NF-EKL, which contributed significantly to the coatings’ ultimate homogeneity and performance. Figure S16 depicts particle patterns and shapes as determined by SEM analyses.
Fig. 4Particle size distributions (10 repetitions) of SF-EKL (top) and NF-EKL (bottom) by laser diffraction particle size analysis
Full size imageCoatings formulations and film formationA high-speed disperser was used to create a variety of coating compositions. The epoxidized lignin was produced as a particulate solid resin that was xylene insoluble. As a result, the coating compositions were made up of a mix of conventional epoxy resins, hardener, xylene, and dispersed epoxidized lignin resin particles. Curing and drying at room temperature allowed for resemble procedures commonly employed in industry. This, however, raises the possibility of vitrification and post-curing consequences as shown in the thermal properties paragraph. Except for the coating containing NF-EKL, which had a rough agglomerated topography, all of the dried and cured films had a smooth surface, as illustrated in Fig. 6. According to scheme 2, the curing process was the result of a combination between physical drying by solvent evaporation and a chemical crosslinking.
Scheme 2