Hyperbranched Polymer Assisted Curing and Repairing of an Epoxy

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Materials and Interfaces

Hyperbranched Polymer Assisted Curing and Repairing of an Epoxy Coating Jiarui Han, Tuan Liu, Shuai Zhang, Cheng Hao, Junna Xin, Bao-Hua Guo, and Jinwen Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00800 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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Industrial & Engineering Chemistry Research

Hyperbranched Polymer Assisted Curing and Repairing of an Epoxy Coating

Jiarui Han,‡ Tuan Liu,‡, Shuai Zhang, Cheng Hao, Junna Xin, Baohua Guo,* Jinwen Zhang*

J. Han, Prof. B. Guo Key Laboratory of Advanced Materials of Ministry of Education, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China E-mail: [email protected]

Dr. T. Liu, Dr. S. Zhang, C. Hao, Dr. Junna Xin, Prof. J. Zhang School of Mechanical and Materials Engineering, Composite Materials and Engineering Center, Washington State University, Pullman, WA 99164, USA E-mail: [email protected]

‡ J. Han and T. Liu contributed equally to this work.

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Imparting repairability to the thermoset coating without compromising other properties is desirable. In this work, a catalyst-free and repairable epoxy coating based on vitrimer chemistry was developed by using a mixture of bisphenol A epoxy (DER) and a hyperbranched epoxy (HBE) as matrix, and succinic anhydride (SA) as curing agent. The abundant hydroxyl groups in the HBE accelerate both the rates of curing and the dynamic transesterification. The presence of DER epoxy ensures the low viscosity of the coating, and its high epoxy value results in the formation of sufficient ester bonds after curing. When the epoxy curing system is used as coating for tin plate, the coating shows excellent hardness and adhesion properties. The scratch on the surface can be repaired at 150 °C without external pressure. The repaired coating well protects the metal substrate from corrosion in NaCl aqueous solution.

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INTRODUCTION Conventional thermosets cannot be repaired and reprocessed like thermoplastics. To reduce the waste and extent the service life of thermoset materials, researchers have made a great effort on developing “repairable” and/or “reprocessable” crosslinked polymers based on covalent adaptable networks (CANs).1-2 When subjected to external stimuli (heat3-4, light5, etc.), the covalent bonds in CANs undergo either the dissociative (reversible covalent, e.g., retro Diels-Alder reaction)6-7 or associative (bond-exchanging)3-4,

8-11

process. The

crosslinked polymers based on the dissociative mechanism are more amendable to recycling as they are reverted back to the thermoplastic-like or even oligomer/monomer state at the processing temperature11-12, while those based on the associative mechanism retain crosslinked structure and exhibit repairability and shape adaptivity.13 The latter networks were termed as “vitrimer” by Leibler group.14 Epoxy vitrimers based on the dynamic transesterification (DTER) mechanism have received the most extensive investigation,15-18 which is partly because the transesterification mechanism can be conveniently applied to many polymers especially the widely used epoxy/anhydride (or carboxylic acid) resin systems.19-21 Scheme 1a shows the mechanism of DTER in the crosslinked epoxy vitrimer. At elevated temperatures (> 150 °C), the transesterification between ester bonds and hydroxyl groups proceeds dynamically, which results in the reconstruction of the crosslinked network and provides certain malleability to the materials.22-25 However, most epoxy vitrimers in the literature highly rely on the use of catalyst for both curing reaction and bond exchange reaction,26-27 and the catalyst usually amounts 5 mol% or higher on the basis of epoxy or -COOH groups to ensure fast repairing 3



and shape changing properties of the crosslinked polymer.28 The catalysts are usually organic salts29-31 or strong bases22,

32;

and some of them are toxic and poorly soluble in polymer

matrix. The addition of catalysts that contain metal ions could also promote corrosion to the substrates especially when the vitrimer material is used as coating or adhesive. In addition, there are also concerns about the long-term environmental stability, such as catalyst leaching and hydrolysis of the material.33 Several non-epoxy based vitrimers which require no catalyst for the dynamic bond-exchange reactions like reversible aromatic disulfides, swift transalkylation reactions, and transcarbamoylation reactions have been developed.34-41 However, catalyst-free epoxy vitrimers based on DTER are rarely seen in the literature. Altuna et al. reacted epoxidized soybean oil and citric acid (Epoxy/-COOH equivalent ratio = 1/0.5 to 1/1) in aqueous solution without catalyst, and the resulting crosslinked polymers exhibited stress relaxation and self-healing properties.42 The drawbacks are that the prepared materials were very soft, and the large amount of water from the citric acid solution should be evaporated carefully during the sample preparation. Recently, we introduced a catalyst-free epoxy vitrimer based on a hyperbranched epoxy (HBE) prepolymer and an anhydride monomer.43 The abundant hydroxyl groups in HBE (Scheme 1b) displayed the catalyzing effect in addition to serving as reacting moieties in both curing and the DTER processes.44 Compared with many liquid BPA epoxies (e.g., DER 331), the HBE in that study exhibited significantly higher viscosity (8400 Pa·s in HBE vs. 0.6 Pa·s in DER 331 at 60 °C), which may limit its applications. In addition, the preparation of HBE may add additional cost to the final epoxy product. It is understood that hyperbranched polymers are often used as modifiers (i.e. toughener, reinforcement, and 4


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additive for reducing dielectric constant, etc.) rather than bulk matrices in epoxy preparation.45-50 Additionally, the HBE based on the polyether structure had a much lower epoxy value than BPA epoxies. After curing with anhydride monomer, the cured product possessed insufficient ester bonds, which limited the rate of DTER and hence the repairing rate of the materials. In our previous work, it required more than 1 x 104 s for the HBE-1/SA material to relax at 160 °C.43 In this study, a fast repairing and catalyst-free epoxy vitrimer was demonstrated by using a mixture of bisphenol A epoxy and HBE as matrix. A low viscosity bisphenol A epoxy resin (DER 331) acted as a diluent to allow low viscosity for the resin system. The reaction between DER 331 and anhydride monomer resulted in formation of considerable ester bonds which would provide sufficient one of the reacting moieties for DTER. The HBE provided another reacting moiety with its abundant hydroxyl groups which also exhibited a clear catalyzing effect in both curing and the DTER processes. At a DER/HBE mass ratio of 1/1, the cured materials showed the fastest relaxation rate. More importantly, the use of the DER/HBE curing system as a fast repairing coating was demonstrated.

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Scheme 1 (a) Schematic illustration of the dynamic transesterification (DTER) in epoxy vitrimer; (b) Synthetic route of hyperbranched epoxy (HBE).

EXPERIMENTAL SECTION Materials

DER 331 epoxy resin (epoxy value is 0.52-0.54 mol/100 g) was provided by the

Olin company. Trimethylolpropane (TMP, TCI, 98%), tetrabutylammonium bromide (TBAB, TCI, > 98%), succinic anhydride (SA, Acros Organics, 99%), zinc acetate (Acros Organics, > 99%, anhydrous), sodium hydroxide (NaOH, MACRON), sodium chloride (Acros Organics, > 99%), hydrochloric acid (HCl, Beijing Chemical Works, 36.0-38.0 wt%), and phenolphthalein indicator (RICCA, 0.5% w/v Alcohol) were used as received. All solvents (GR grade) were used without further purification.

Synthesis of hyperbranched epoxy (HBE) prepolymer

HBE (epoxy value is ~0.26

mol/100 g) was synthesized according to our previous work.43 HBE was synthesized by the reaction of DER 331 (A2 monomer) and TMP (B3 monomer) through a simple one-step reaction without any solvents. The molar ratio between the epoxy groups of DER 331 and 6


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hydroxyl groups of TMP was fixed at 2:1 in order to make sure that the terminal groups of HBE are mainly epoxy groups. In detail, 100 g DER 331 and 11.4 g TMP were charged into a 250 mL three-necked flask and mixed at 50 °C with magnetic stirring. When a transparent mixture was formed, 8.54 g TBAB (5 mol% on the basis of epoxy groups) as catalyst was added. The reaction was performed at 120 °C for 48 h under N2 atmosphere. After reaction, the crude HBE was purified by dissolving the mixture in 100 mL chloroform and then pouring the solution into 1000 mL methanol. The precipitate was washed three times with methanol to remove the catalyst and unreacted monomers and then dried in vacuum oven at 60 °C to give a product (yield: 61%). It is worth stressing that the reaction of one epoxy group and one primary -OH group from TMP generates a new secondary -OH group during polymerization. Because the reactivity of the secondary -OH group is much lower than that of the primary -OH group, epoxy groups mainly react with TMP (Scheme 1). The structure of the HBE was confirmed by 1H NMR (400 MHz, CDCl3, δ): 6.81, 7.12 (CH, aromatic); 4.16-3.48 (CH and CH2); 3.33 (CH, epoxide); 2.88 (CH2, epoxide); 2.73 (CH2, epoxide); 1.62 (CH3, polymer backbone); 0.81, 1.28 (CH and CH2, polymer backbone). HBE has a weight-average molecular of 9334 Da and an epoxy value of 0.26 mol/100 g.

Preparation of HBE/DER-SA and HBE-SA vitrimers HBE and the mixture of HBE and DER331 were cured with SA according to the formulations listed in Table 1. The typical procedures are as follows. First, HBE and its mixture with DER331 in a certain weight ratio (i.e. 1/3, 1/1, 3/1) were heated at 50 °C to form a homogeneous solution. The solution was heated to ~130 °C, and then SA was added. The high mixing temperature is to facilitate the 7



melting of SA, whose melting temperature is ~120 °C. The molar ratio of epoxy group and anhydride group (E/A) was fixed at 2:1. After a homogeneous HBE/DER/SA solution was formed, it was quickly transferred to a Teflon mold with a dimension of 100 mm × 150 mm × 0.3 mm and cured in a hot-press. A three-step heating process was used for curing: 120 °C for 30 min, 160 °C for 2 h, and 180 °C for 1 h. After curing, the film was cooled down naturally to room temperature.

Preparation of vitrimer coating and testing The vitrimer coating was prepared via curing of epoxy resin on standard tin plates. Before coating, the surface of the tin plate was polished with a 400 grits abrasive paper to remove the protective layer and washed with an alkaline solution (5 wt% NaOH solution). Subsequently, the tin plate was washed with ethanol and acetone successively and then dried in a convection oven at 30 °C. The pretreated tin plate was placed on a digital hot plate (Corning PC-420D) with a temperature of 130 °C. The curing system was then coated on the surface of the tin plate with a film applicator to control the thickness. The coating was cured at 120 °C for 30 min, 160 °C for 2 h, and 180 °C for 1 h in a convection oven. After curing, the coating was cooled naturally to room temperature. For comparison, the DER-SA-CAT coating which contains 10 mol% zinc acetate (based on SA) was also prepared following the similar above procedures. The thickness of the coating layer was measured using a thickness gauge according to the following steps. First, the thickness of the uncoated tin plate was measured. Three points on the tin plate were selected and measured. After coating process, the thickness of the same selected three points were measured. The thickness difference before and after coating of the 8


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three selected points was averaged to be the thickness of the coating layer. The hardness of the coating was measured according to ASTM D3363-05, a Standard Test Method for Film Hardness by Pencil Test. First, the coated tin plate was placed on a horizontal surface. Starting from the hardest pencil level, the pencil was hold against the coating at a 45° angle and pushed away from the tester with sufficient uniform pressure. The process was repeated until a pencil was found that cannot cut through the coating to the substrate for at least 3 mm distance. This pencil scale was recorded as the gouge hardness. The test was then continued until a pencil was found that neither cut through nor scratch the surface of the coating. This pencil scale was recorded as the scratch hardness. The adhesion of the coating was determined according to ASTM D 3359-17, a Standard Test Methods for Rating Adhesion by Tape Test (Test Method B-Cross-cut Tape Test). First, a 10×10 grid with a spacing of 1 mm was cut by a sharp razor blade. There must be sufficient pressure to make the cuts reach the substrate. The test tape was then placed over the grid and bonded tightly with the coating. The tape was torn off rapidly from the coating by seizing the free end. The adhesion level was determined based on the damaged area from the tape test. For examples, 5B level indicates no coating was removed, and 0B level indicates more than 65% of the coating was moved by tape.

Repairing property The coating was scratched using a razor blade to make a crack on the surface. The scratched coating was then repaired by heating in a convection oven. The effect of time (ranging from 5 to 60 min) on repairing were investigated. The coating was repaired at 150 ° C. The width and the surface morphology of the crack during repairing was 9



monitored by optical microscopy. For the repairing of film sample, the crack on the film was monitored by scanning electron microscopy (SEM). An accelerating voltage of 3 kV was applied, and the sample was sputter coated with platinum prior to the SEM measurement. Electrochemical corrosion test was applied to examine the corrosion resistance of the repaired coating. Figure S1 shows the illustration of the device for the electrochemical corrosion test. The coating sample was acted as working electrode, the platinum electrode was the counter electrode, and the saturated calomel electrode was the reference electrode. All the three electrodes were immerged in a 5 wt% NaCl aqueous solution which was served as electrolyte solution. The test was performed at room temperature under a voltage of 3 V. The current as a function of time was recorded by an electrochemical station (CHI 660e).

Characterizations The viscosity of the resin was measured on a Discovery HR-2 rheometer (TA Instruments) with a 25 mm parallel plate geometry. The sample was scanned from 130 to 30 °C with a shear rate of 1 s-1. The data was recorded every 10 °C during measurement. The curing behavior were studied by differential scanning calorimetry (DSC1, Mettler-Toledo. Switzerland). The sample (3-5 mg) was sealed in an aluminum crucible and heated from 25 to 250 °C at different heating rates (2, 5, 10, and 15 °C/min) under N2 atmosphere. The Fourier transform infrared (FTIR) spectra of the cured samples were collected on a Thermo Scientific Nicolet 6700 FTIR spectrometer in ATR mode. Before testing, the sample was placed in a vacuum oven at 80 °C overnight to remove the moisture. The dried sample was scanned from 400 to 4000 cm-1 for 64 scans with a resolution of 4 cm-1. The gel content and swelling ratio of the cured samples were tested according to ASTM 10


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Standard D2765-16. The sample (Ws) was immersed in xylene and heated at 110 °C under continuous stirring for 24 h. After cooling to room temperature, the swollen sample was quickly transferred to a dry and weighed vial and weighted (Wg). The swollen sample was dried in a vacuum oven at 100 °C to completely remove the xylene until a constant weight was reached, and the dry weight of the extracted sample was recorded as Wd. The swelling ratio and extract were calculated according to the following equations:

Swelling ratio =

Extract, % =

𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑝𝑜𝑙𝑦𝑚𝑒𝑟 𝑖𝑛 𝑔𝑒𝑙 + 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑥𝑦𝑙𝑒𝑛𝑒

𝑊𝑠 ― 𝑊𝑑 𝑊𝑠

𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑝𝑜𝑙𝑦𝑚𝑒𝑟 𝑖𝑛 𝑔𝑒𝑙

× 100

=1+

𝜌𝑃𝑜𝑙𝑦𝑚𝑒𝑟(𝑊𝑔 ― 𝑊𝑑) 𝜌𝑆𝑜𝑙𝑣𝑒𝑛𝑡𝑊𝑑

(1)

(2)

The thermal stability was determined by a thermogravimetric analyzer (TGA, TGA/DSC1, Mettler-Toledo). The sample (~10 mg) was loaded into a 70 μm ceramics crucible and scanned from 50 to 800 °C at a heating rate of 10 °C/min under N2 atmosphere. The dynamic mechanical properties were determined by a dynamic mechanical analyzer (DMA, Q800, Thermal Instrument) with film tension mode. Samples with a dimension of 15 mm × 4 mm × 0.3 mm were scanned from -20 to 150 °C at a heating rate of 3 °C/min. The oscillating amplitude was set at 15 μm, and the frequency was set at 1 Hz. The isothermal stress relaxation was measured by DMA instrument with film tension mode. The sample with a dimension of 15 mm × 4 mm × 0.3 mm was heated at the test temperature (i.e. 120, 140, 160, and 200 °C) and equilibrated for 5 min. A 4% stain was applied to the sample, and the relaxation modulus as a function of time was recorded. 11



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RESULTS AND DISCUSSION Viscosity of the mixed epoxy and curing behavior

We

previously

demonstrated

a

catalyst-free vitrimer system based on HBE. In this work, HBE was blended with DER epoxy in mass ratios of 3/1, 1/1, 1/3, respectively, for the coating application. HBE exhibited a Newtonian fluid behavior (Figure S2), as its viscosity was almost constant at different shear rate from 0.01 to 10 s-1. Figure 1a shows the viscosities of the epoxy blends as functions of temperature. HBE is a solid at room temperature and turns into a liquid by heating. At 100 °C, the viscosity of HBE is ~90 Pa·s which is much higher than that of DER epoxy (< 1 Pa·s, 100 °C). Therefore, HBE alone as a matrix is not convenient for many applications that require low curing temperature and excellent flowability of the resin. The mixing of DER epoxy with HBE significantly lowered the viscosity as shown in Figure 1a. At a 1/1 ratio, the viscosity of HBE/DER dropped to 2 Pa·s at 100 °C which was about the same level to that of DER epoxy resin.

Figure 1 (a) Viscosity of HBE/DER systems as a function of temperature; (b) Fitting curves of Tgel, Tcure, Tpost-t in HBE/DER-1/1-SA system obtained by Ti, Tp, Tf characteristic 12


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temperatures at heating rates of 2 °C/min, 5 °C/min, 10 °C/min and 15 °C/min, respectively.

Succinic anhydride (SA) was selected as curing agent to react with HBE and DER epoxy resin, and the molar ratio of epoxy to anhydride was fixed at 1:0.5. Figures S3 - S6 show the DSC exothermic curves of the resin systems with different compositions at the heating rates of 2, 5, 10 and 15 °C/min, respectively. From the curves, the exothermic initial temperature (Ti), exothermic peak (Tp) and exothermic final temperature (Tf) at different heating rates were obtained. The Ti, Tp, Tf were plotted against the heating rate (Figure 1b), and the linear fittings were extrapolated to the temperature at β = 0 °C/min for each curve. The Ti, Tp, Tf at β = 0 °C/min are defined as gel temperature (Tgel), curing temperature (Tcure) and post-treatment temperature (Tpost-t), respectively. Following the same method, Tgel, Tcure, Tpost-t for all systems were calculated and listed in Table 1. Tgel, Tcure, Tpost-t decreased as the loading of HBE increased. This is because the hydroxyl groups from HBE efficiently induced the ring opening of SA for further crosslinking, so the curing reaction could happen at lower temperature with a higher HBE loading. It is also noted that the curing reaction did not happen in the absence of HBE below 200 °C as shown in Figure S7. This is because DER epoxy contains very few hydroxyl groups to induce the curing, so the curing reaction cannot happen. In considering both viscosity and curing factors, a three-step curing was applied, including a thermal pretreatment at 120 °C for 0.5 h, thermal treatment at 160 °C for 2 h, and thermal post-treatment at 180 °C for 1 h. Table 2 shows the gel content and swelling ratio of the cured products. HBE-SA, HBE/DER-3/1-SA, and HBE/DER-1/1-SA all exhibited a gel content higher than 90%, 13



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indicating a high degree of crosslinking was achieved. In contrast, the gel content of HBE/DER-1/3-SA was ~87.2%, because the relatively low loading level of HBE could not provide sufficient hydroxyls for inducing the crosslinking and thereby resulting in a low gel content. In addition, it is noted the swelling ratio of HBE/SA (~1.18) was much lower than that of HBE/DER-SAs (1.4 - 1.5). HBE possesses multiple hydroxyl and epoxy groups which make it prone to form highly crosslinked structure and exhibit low swelling ratio.

Table 1 Formulations and curing behaviors of curing systems Sample

a

HBE (g) DER (g)

SA (g)

Tgel (°C)

Tcure (°C)

Tpost-t (°C)

HBE-SA

3.0

0.0

0.39

97.5

170.5

187.0

HBE/DER-3/1-SA

2.1

0.7

0.46

102.8

171.2

189.3

HBE/DER-1/1-SA

1.5

1.5

0.59

103.9

175.5

189.7

HBE/DER-1/3-SA

0.7

2.1

0.65

105.2

180.1

194.1

DER-SA-SATa

0.0

3.0

0.80

-

-

-

10 mol% Zinc(II) acetylacetonate hydrate was added on the basis of SA.

Dynamic mechanical properties Figures 2a and 2b show the storage modulus (E') and tan δ as a function of temperature, respectively. For each composition, E' showed a rapid drop from glassy state to rubbery state. With increase in DER content, the moduli of both glassy state and rubbery state decreased. The modulus of crosslinked polymer is closely related to the crosslink density. HBE contains abundant reactive hydroxyl groups and epoxy groups.

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The addition of DER 331 lowered the average functionality of the resin system and decreased the crosslink density, which finally caused a reduction in both glassy modulus and rubbery modules (Table 2). This result is consistent with the extraction results for gel contents. The glass transition temperature (Tg) were obtained from the peak temperature of tan δ curves. The Tg decreased with the increase of DER epoxy content in the mixed epoxy (Table 2), which was due to the decrease in crosslink density. HBE-SA showed a Tg of 82.5 °C, in contrast, HBE/DER-1/3-SA exhibited the lowest Tg of 60.5 °C among all samples.

Figure 2 (a) Storage modulus (E') and (b) tan δ of the cured HBE/DER-SA systems; (c) TGA and (d) DTG curves of the cured HBE/DER-SA systems.

Table 2 Thermal and Swelling properties of systems with different HBE/DER ratios 15



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Thermal 𝐸′𝑔𝑎

𝐸′𝑟𝑏

Swelling

(MPa)

(MPa)

ratio

Gel content Tgc

Sample (%)

degradation (°C)

(°C) Ti

Td5

Tmax

HBE-SA

3350

16.7

1.18±0.08

95.5±0.9

82.5 283 376

420

HBE/DER-3/1-SA

3100

12.8

1.40±0.01

95.4±2.3

78.7 249 359

416

HBE/DER-1/1-SA

2730

3.7

1.43±0.01

91.1±3.2

65.4 247 338

412

HBE/DER-1/3-SA

2470

3.1

1.49±0.05

87.2±2.4

60.5 180 313

410

aE ′ g

was obtained at 30 °C; bEr′ was obtained at 110 °C; cTg was obtained from the peak

temperature of tan δ curves.

Thermal stability

Figures 2c & 2d show the TGA curves of the curing systems. The

initial weight loss temperatures of all compositions were above 200 °C, indicating their fairly good thermal stability. It is noted that the temperature at 5 wt% of weight loss (Td5) decreased with increase in DER epoxy content, which could be partially related to the decrease in crosslink density. Because DER 331 (epoxy value = 0.52 - 0.54 mol/100 g) has a much higher epoxy value than HBE (epoxy value = ~0.26 mol/100 g), the increase of DER epoxy content in the mixed epoxy resin would require increase loading of SA and hence result in the formation of more ester bonds in the crosslinked network. This postulation was proved by the FTIR spectra in Figure 3a, where the intensity of the peak at ~1735 cm-1 attributed to the ester bonds increased with DER epoxy loading. Compared with ether bonds, ester bonds are less thermally stable and would contribute to the decreased thermal stability of the 16


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crosslinked polymers.

Figure 3 (a) FTIR spectra of the cured HBE/DER-SAs, in which each spectrum was normalized against to the peak of benzene skeleton at ~1607 cm-1 as the internal standard; (b) Area-OH/Area-ester of the cured HBE/DER-SAs.

Stress relaxation Thermal stress relaxation is a test that examines the sample in response to an applied constant strain. Figure 4 shows the relaxation curves at 160 °C. The relaxation time is defined as the time when the relaxation modulus reaches to 1/e of its initial modulus. Compared with the systems containing HBE, the DER-SA-CAT system showed much slower relaxation rate, and its relaxation modulus decreased for only ~10% in 200 min. Among all samples, HBE/DER-1/1-SA exhibited the fastest stress relaxation rate with a relaxation time of 68 min which was less than one tenth of that of HBE-SA. It is noted the relaxation time first decreased and then increased with the increase in DER epoxy content. This phenomenon was probably due to the following two reasons. On one hand, the decreased crosslink density associated with the loading of DER epoxy rendered the flexibility of the polymer chains and promoted the DTER and relaxation rate. On the other hand, the increase in the loading of 17



DER tend to form more ester bonds but less hydroxyl groups after curing. The slow relaxation for HBE/DER-1/3-SA was due to the insufficient hydroxyl groups in network. To evaluate the contents of hydroxyls and ester bonds in network, the area of the peak at ~1735 cm-1 attributed to ester bonds and the area of the peak at 3400 cm-1 attributed to hydroxyl groups in the FTIR spectra were compared (Figure 3b). The Area-OH/Area-ester ratio of the HBE-SA system was 1.95. As the loading of DER epoxy increased, the Area-OH/Area-ester ratio decreased, indicating less hydroxyl groups and more ester bonds present in the crosslinked network. For the cured HBE/DER-1/1-SA, a proper Area-OH/Area-ester ratio of 0.92 exhibited fast relaxation.

Figure 4 Stress relaxation curves of HBE/DER-SAs, HBE-SA and DER-SA-CAT at 160 °C.

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Figure 5 Optical micrographs of the scratched coatings heated at 150 °C for different time intervals. The width of the scratch was measured at the same location for each sample.

Figure 6 SEM images of the scratched HBE/DER-1/1-SA film (a) before and (b) after repairing at 150 °C for 1 h.

19



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Coating properties Epoxy materials are widely used as coatings due to their excellent mechanical properties, chemical resistance, etc. However, conventional epoxy coatings cannot be repaired easily due to their permanent crosslinked network. Imparting repairability to epoxy coating without compromising other performances is desirable. In this work, the viscosity of the resin system was adjusted by manipulating the ratio of DER epoxy and HBE to be appropriate for coating application. (1) Hardness and adhesion properties Table 3 shows the pencil hardness and adhesion properties of HBE/DER-SA coatings. The layer of HBE-SA coating was thicker than others, which was because the high viscosity and fast curing rate made it difficult to form thin coating. All compositions displayed excellent coating hardness with the gauge hardness above 6H. In the ASTM pencil test, the highest hardness level was 6H. The coatings in this work can withstand even harder pencil (the hardest pencil is 9H) scratches than 6H pencil, indicating that our coatings have excellent hardness. In addition, the coatings on metal surface showed excellent adhesion, and no area was peeled off during the crosshatched test. All the HBE/DER-SA coatings exhibited comparable hardness and adhesion properties to DER-SA-CAT coating.

Table 3 Coating properties with different HBE/DER ratios Thickness

Gouge

Scratch

Adhesion

（μm）

Hardness

Hardness

Test

80±5

>6H

4H

5B

Sample

HBE-SA

20


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HBE/DER-3/1-SA

48±5

>6H

4H

5B

HBE/DER-1/1-SA

43±3

>6H

6H

5B

HBE/DER-1/3-SA

20±2

>6H

6H

4B

DER-SA-CAT

36±5

>6H

>6H

4B

(2) Repairing property The DTER leads to the reconstruction of the crosslinked network and provides repairing property to the vitrimer coating. Figure 5 shows the change of the crack width with time during the repairing of coatings. All coating samples exhibited a decent repairability, as the width of the crack was reduced over 50% within 10 min for each sample. After 60 min, the scratch on the surface of the HBE/DER-1/1-SA coating sample almost completely disappeared, while that of the other samples also recovered more than 67%. It is noted the fast repairing of HBE/DER-1/1-SA was achieved under the condition without external pressure, indicating that the scratches of the HBE/DER-1/1-SA could be repaired simply with a conventional heater. Figures 6a and 6b show the scratched HBE/DER-1/1-SA film before and after repairing, respectively. The scratch recovery was examined by SEM. The scratch almost disappeared after heating treatment, indicating its excellent repairing property. In contrast, the width of the scratch from DER-SA-CAT film recovered for only ~10% after heating at 150 °C for 90 min. It is noted that the repairing of HBE/DER-1/1-SA film (Figure 6) is faster than that of HBE/DER-1/1-SA coating (Figure 5). This is because the tin plate limits the deformability of the coating, the repairing of coating is slower than that of film sample. 21



(3) Corrosion resistance test

Because HBE/DER-1/1-SA showed the best reparability,

it was used to investigate the corrosion of the coating and I-t test. The scratch was prepared by cutting through the HBE/DER-1/1-SA coating to the metal substrate using a razor blade. The scratched coating was repaired at different temperatures (i.e., 120, 140, 160, 180 °C) and time intervals (i.e., 30, 60, 120, 200 min), respectively. During repairing, the coated tin plate was covered with another tin plate, clamped with a clip, and heated in a convection oven. The scratched coating without repairing was used as a control. For the corrosion test, all the coating samples were immerged in a 5 wt% NaCl aqueous solution for 360 h at room temperature. Figure 7 shows the samples after immersion. For the control sample (Figure 7i), the scratched coating was significantly corroded. For the repaired coating samples, the corrosion resistance of the repaired coating improved (less eroded area) with the increase of repairing temperature (Figure 7a-c, h). After the coating repaired at 160 °C for over 60 min, almost no corrosion was observed on the sample. In contrast, for the sample repaired at 120 °C at which the rate of DTER was not high enough to bring good repairing of the crack to isolate the substrate from the NaCl aqueous solution, a serious corrosion was noted. The effect of repairing time on corrosion resistance was also investigated. The coating repaired at 160 °C for 30 min (Figure 7f) still showed slight corrosion, but no corrosion was noted anymore when the repairing time was increased to 200 min (Figure 7c). For comparison, the corrosion resistance of the DER-SA-CAT system was investigated. The scratched DER-SA-CAT coating was treated at 160 °C for 200 min (Figure 7j). After immersed in the salt solution for 360 h, the coated tin plate was seriously corroded (Figure 7k). This result 22


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indicates the repairing of DER-SA-CAT was not as efficient as that of HBE/DER-1/1-SA. Furthermore, the presence of a significant amount of zinc catalyst in DER-SA-CAT probably also promoted the corrosion.

Figure 7 (a)-(h) corrosion digital photos of self-healed scratched coating regions of HBE/DER-1/1-SA system under different repairing conditions and (i) control scratched coating region of HBE/DER-1/1-SA after 15 days in 5 wt% NaCl solution; (j) and (k) corrosion digital photos of self-healed (160 °C, 200 min) and control scratched coating regions of DER-SA-CAT system after 15 days in 5 wt% NaCl solution. Electrochemical testing provides another evidence for the reparability. As shown in Figure 8, a strong current ranging from 23.1 mA/cm2 to 79.4 mA/cm2 was detected for the scratched HBE/DER-1/1-SA coating. As the repair temperature increases, the conductivity at the scratch gradually decreases. After repairing at 160 °C for 200 min, there was weak current 23



(2×10-4 μA/cm2) was detected, which indicates that the repaired coating well protects the tin plate from electrochemical corrosion.

Figure 8 Current-t curves of the coatings repaired at different temperatures.

CONCLUSIONS An epoxy vitrimer system which demonstrated a promising potential for the application of repairable coating was introduced. The epoxy vitrimer material was synthesized by curing a mixed epoxy resin consisting of a commercial bisphenol A epoxy and a hyperbranched epoxy introduced in this study. Inclusion of HBE enabled to eliminate the need of catalyst in the preparation of the vitrimer material, because the abundant hydroxyl groups in HBE catalyzed both the curing of epoxy and subsequent transesterification in the network at the elevated temperature. While the inclusion of bisphenol A epoxy ensures the low viscosity of the polymer matrix, and its high epoxy value results in the formation of sufficient ester bonds after curing with anhydride monomer. At a weight ratio of 1/1, the viscosity of HBE/DER (2 Pa·s at 100 °C) was about the same level to that of neat DER. The crosslinked 24


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HBE/DER-1/1-SA possessed abundant hydroxyls and ester bonds for DTER. The relaxation time of HBE/DER-1/1-SA, which is an indicator of DTER, was 67 min in contrast to 775 min for HBE-SA. HBE/DER-SAs were used as coating for tin plate. All the coatings showed excellent hardness and adhesion when applied to the tin plate. HBE/DER-1/1-SA coating exhibited the best repairing property due to its fast DTER, and the scratch on the surface of HBE/DER-1/1-SA coating can be recovered for over 90% at 160 °C within 1 h under a convenient condition without external pressure. The repaired coating well protected the metal substrate from the corrosions of electrochemical and salt solution. This work reported a fast repairing epoxy coating based on vitrimer chemistry and may provide a way to extent the service life and reduce the waste of thermosetting polymers.

ASSOCIATED CONTENT Supporting Information The Supporting Information includes Figures S1-S8

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (J.Z.) and [email protected] (B.G) Author Contributions ‡ Jiarui Han and Tuan Liu contributed equally to this work. Notes Notes The authors declare no competing financial interest. 25



Jiarui Han is a visiting student at Washington State University,

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant No. 51673110, 51473085) and the Joint Funds of the National Natural Science Foundation of China (Grant No. U1862205).

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Table of Content This work introduces a fast repairing and high performance epoxy coating based on vitrimer chemistry and may provide a way to extent the service life and reduce the waste of thermosetting polymers.


Hyperbranched Polymer Assisted Curing and Repairing of an Epoxy

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