Development of waterborne epoxy-based resin incorporated with modified natural rubber latex for coating application | Scientific Reports

Scientific Reports volume 14, Article number: 26603 (2024) Cite this article

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Water-based coating has gained much attention globally due to environmental issues. This work aims to develop a waterborne epoxy coating incorporated with modified natural rubber (NR) latex for improved performance. For this purpose, the NR latex was modified into three types of low molecular weight epoxidized natural rubber (LENR) latex. The waterborne epoxy resin was prepared by simply mixing the epoxy resin with an amine curing agent in water and various LENR latex contents (10 to 50 wt%). The waterborne epoxy/LENR blend was coated on a tin substrate and the cured coating demonstrated better adhesion and bending properties than the neat epoxy. Phase morphological properties displayed a rough surface with a heterogeneous surface structure of the dispersed rubber and epoxy matrix. Two glass transition temperatures were observed, corresponding to the rubber modifier and the epoxy matrix. The mechanical properties of the thermosetting epoxy/LENR blends were assessed, revealing a maximum increment in elongation ability by approximately 370% for adding 50 wt% LENR, compared to the neat epoxy. These results hold the potential avenue for utilizing NR-based material in the waterborne epoxy-based resin for coating applications. Furthermore, it contributes to the NR, a renewable and eco-friendly material, in the coating technology, embracing a sustainable future.

Since the past few decades, the rapid increase in global average temperature has gained substantial attention worldwide. Several international summits have discussed environmental regulations for reducing greenhouse gas emissions. The volatile organic compounds (VOCs) in petrochemical, paint and coating industries have been considered one pollutant emission, contributing to environmental and health hazards1,2. Consequently, several countries in different parts of the world are greatly concerned about implementing regulations to limit the number of VOCs in polymer coating technology, aiming to reduce VOC emissions3,4,5. In response to this effect, a waterborne coating system utilizing water instead of a volatile organic solvent has been increased up to 80% market share6. The remarkable aspects of waterborne coatings include non-toxicity, non-flammable behavior, availability and low VOC emission7,8. Among waterborne coating systems, the epoxy-based coating has the highest volume in the market due to its outstanding properties of the cured products, such as excellent adhesion and bonding properties, good corrosion resistance, excellent thermal stability and good chemical stability9,10. Nevertheless, the highly crosslinked epoxy materials are susceptible to brittle failure, hindering their applications in some fields11,12. Several synthetic elastomers such as carboxyl-terminated polybutadiene13, carboxyl-terminated poly(butadiene-co-acrylonitrile)14, carboxyl-terminated acrylate rubber15, hydroxyl-terminated polybutadiene12, and styrene butadiene rubber (SBR)16 have been employed for toughening modification of the epoxy-based thermosetting material. Finding an alternative substance like natural rubber that is eco-friendly is therefore challenging.

Natural rubber (NR), a renewable elastomeric material obtained from the rubber tree (Hevea brasiliensis) in latex form, is a potential eco-friendly candidate as a toughening agent for waterborne epoxy-based coating. A cis-1,4-polyisoprene structure can be chemically modified in the latex phase for different functional groups17. The utilization of NR and its derivatives has been achieved as a mechanical modifier to improve the impact resistance of brittle materials, such as poly(vinyl chloride)18,19, poly(lactic acid)20,21 and UV-curable resin applied in DLP-3D printing technology22,23. For epoxy-thermoset polymers, NR-based materials have been explored as toughening agents of the epoxy materials. Ruamcharoen et al.24 synthesized the ENR latex having various epoxide contents (20, 45 and 60%) and blended with epoxy resin in the range of 2 wt% to 6 wt%. They revealed that the epoxy blended with 2 wt% of ENR latex having 60% epoxide content provided proper mechanical strength and extensibility, resulting from compatibility between the ENR phase and the epoxy matrix. Yong et al.25 reported the preparation of natural rubber-based epoxy resin by reducing the molecular weight of epoxidized natural rubber consisting of 25% epoxide content in the tetrahydrofuran (THF) solvent through ultraviolet (UV) treatment. The prepared epoxy resin solution (35 wt%) was thoroughly blended with pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) as a hardener in THF to form the coating product. As a result, the film coating showed excellent adhesion, hardness, and chemical resistance with a reasonably good thermal decomposition. Nevertheless, this typical natural rubber-based epoxy coating was still prepared in the volatile organic solvent. The utilization of NR-based material in the latex stage for a waterborne epoxy coating has still been scarce. Therefore, it is promising as an alternative to contribute the NR in the coating technology.

In this work, three types of low molecular weight epoxidized natural rubber (LENR) latex were prepared in a facile and eco-friendly system using NR latex as a raw material through a two-step chemical reaction: epoxidation and oxidative degradation reactions. The chemical structure and molecular weight of the modified NR were investigated. The prepared LENR latex was blended with an epoxy resin and amine hardener using a simple mixing process in water. A room-temperature amine curing was used to form the epoxy-based thermosetting product. Subsequently, the coating behavior of the waterborne epoxy/LENR blend, including the conversion, coating properties and mechanical performance, was evaluated. Furthermore, morphological and thermal properties of the coating sample were also conducted to explain the coating manners of the epoxy/rubber blends.

Amine curing agent (Anquamine 721, amine hydrogen equivalent weight (AHEW) of 300 g/eq) was procured from Evonik Ltd. (Bangkok, Thailand). Ammonium hydroxide solution (25%) was obtained from QRec Chemical Co., Ltd. (Auckland, New Zealand). Epoxy resin (Epotec YD 128), a bisphenol-A-based liquid epoxy resin with an epoxy equivalent weight (EEW) of 185 g/eq, was supplied by Aditya Birla Chemicals Ltd. (Rayong, Thailand). Formic acid (85%) and hydrogen peroxide (30%) were procured from Carlo Erba Reagent (Milan, Italy). High ammonia natural rubber latex (HANR, 60% dry rubber content, DRC) was provided by Thai Rubber Latex Co., Ltd. (Samut Prakan, Thailand). Tergitol 15-S-15, a non-ionic surfactant, was purchased from Dow Inc. (Midland, MI, USA). The tin plate was supplied by Richtech Paint Co. Ltd. (Samut Sakhon, Thailand).

LENR was synthesized from NR latex using a two-step chemical modification as demonstrated in Fig. 1. Briefly, stabilized natural rubber latex (60 g of solid rubber, 300 mL) was prepared by diluting HANR (100 mL) with distilled water (200 mL) and then added Tergitol 15-S-15 (3 part per hundred rubber or phr, 1.8 g) following the agitation for 24 h. Subsequently, the slow concurrent addition of formic acid (0.25 mol) and hydrogen peroxide (0.6 mol) into the stabilized NR latex was performed with continuous stirring at 60 °C for 9 h, gaining epoxidized natural rubber (ENR) latex. Upon completion of the epoxidation reaction, the prepared ENR latex was cooled down to room temperature. A small sample was collected for analysis before adjusting the pH to 7 using an ammonium hydroxide solution. Then, Tergitol 15-S-15 (3 phr, 1.8 g) was added to the neutralized ENR latex with further stirring for 15 min, before slowly dropping hydrogen peroxide (0.5, 1.0 and 1.5 mol based on rubber content) at 60 °C with stirring for 24 h to acquire the LENR latex. The LENR latex (14% DRC) was stored in a refrigerator at a temperature of 0–5 °C. A small amount of the prepared LENR latex was coagulated in methanol and then dried for the chemical structure and molecular weight investigation.

Schematic preparation of low molecular weight epoxidized natural rubber (LENR) latex.

The schematic preparation of the waterborne epoxy resin/rubber blend system is illustrated in Figure S1 in Supplementary information using the formulation shown in Table 1. Epoxy resin and amine hardener were weighed and mixed until two components were uniform using an overhead stirrer (Eurostar digital, IKA-Werke, Staufen, Germany) with a rotor speed of 100 rounds per minute (rpm) for 2 min. Afterward, the distilled water was added to the mixture resin and then stirred for 1 min before adding the synthesized LENR latex (14% DRC), following the agitation for 2 min to acquire the homogeneous mixture of the waterborne epoxy resin/LENR blend. The blend was degassed and stayed for 3 min to eliminate the bubbles that occurred during the stirring process.

In the coating process, the prepared waterborne epoxy resin/LENR blends were coated on the tin plate (7.5 cm × 15 cm × 0.35 cm) using a bar coater (BGD 206/3, Biuged Laboratory Instruments Co. Ltd., Guangzhou, China) by controlling the thickness of 50 μm coated sample. The coated products were left at room temperature to form the crosslinked network and dry over various periods (1, 3 and 7 days) before testing.

The attenuated total reflection accessory equipped with Fourier transform infrared (ATR-FTIR) spectra of rubber samples were received from the infrared spectrophotometer (Paragon 1000, PerkinElmer, Waltham, Massachusetts, USA). The measurement was manipulated from 4000 to 400 cm–1 with 64 scans and 4 cm–1 resolution.

Epoxide conversion related to the kinetic profile of the thermoset material was determined by ATR-FTIR mode. The mixture resins before the curing process were evaluated as a reference. Meanwhile, the thin sheet (50 ± 5 μm) of the cured epoxy/rubber blends was measured at six different positions on each side. The conversion values were determined as illustrated in Eq. 126.

where Auncured and Acured are the peak areas of uncured and cured samples at 915 cm[–1 (C-O-C stretching of epoxide group) and 835 cm–1 (= C-H bending of aromatic ring).

The proton nuclear magnetic resonance (1H-NMR) spectra of rubber samples were acquired using a nuclear magnetic resonance spectrometer at a 500-MHz operating frequency (Bruker Corporation, Karlsruhe, Germany). The rubber sample was completely dissolved in deuterated chloroform (CDCl3) as a solvent and tetramethylsilane (TMS) as an internal standard. The chemical shifts were expressed in part per million (ppm). The CDCl3 solvent peak is 7.26 ppm, and the TMS peak is 0.0 ppm. Furthermore, the epoxide content of the modified NRs was determined from the integrated peak area of the spectrum using Eq. 222.

where A2.70 and A5.14 are the integrated peak area of methine protons adjacent to the epoxide ring and methine protons of the double bond of the isoprene unit, respectively. The calculation of epoxide content in each rubber sample was carried out from three different batches, and the mean and standard deviation values were reported.

Gel permeation chromatography (GPC) (150-C ALC/GPC, Waters, Milford, Massachusetts, USA) was utilized to assess the molecular weight of the rubber samples. Approximately 10 mg of rubber samples were completely dissolved in tetrahydrofuran (THF) as a solvent and filtered through a 13-µm nylon syringe filter with a 0.45-µm pore size before measurement. THF as an eluent was fed with a constant flow rate of 1 mL/min at 40 °C. The number average molecular weight (Mn), the weight average molecular weight (Mw) and the polydispersity index (PDI) from three different batches were earned, and the mean and standard deviation values were demonstrated.

The viscosity values of the prepared epoxy/rubber mixture resin were measured using a viscometer (Brookfield Viscometer, DV-II, Brookfield Engineering, Middleboro, Massachusetts, USA). The measurement was carried out with a gel timer spindle (Spindle 4) at a constant speed of 3 rpm for 0–90 min to receive the viscosity profiles of each sample at room temperature (25 ± 2 °C).

The cross-hatch adhesion test of the coating samples was carried out according to ASTM D3359 under room temperature (25 ± 2 °C). The adhesion of the coating layer with tin substrate was investigated by cutting with the right-angle lattice pattern using cross-hatch cutters (BGD 502/2A, Biuged Laboratory Instruments Co. Ltd., Guangzhou, China) having 11 sharp razor blades and 1 mm blades spacing (each cross-hatched square area of 1 × 1 mm2). The test area is then brushed diagonally three times in each direction to eliminate any loose film finish particles. Subsequently, a pressure-sensitive adhesive tape (3 M 600 Scotch® Transparent Tape) was firmly applied over the cross-hatch test area and pressed by a fingertip to obtain uniformity between the adhesive tape and coating surface. The adhesive tape was removed quickly within 90 s by pulling the tape back off the test area. The amount of coating lifted off by the test tape was then visually compared to the standard (ASTM D3359). Each sample of coating products was performed in triplicate.

The surface hardness or the coating’s ability to resist deformation of the coating samples was conducted by pencil scratch method using a pencil hardness tester (BGD 506/3, Biuged Laboratory Instruments Co. Ltd., Guangzhou, China) according to ASTM D3363 under room temperature (25 ± 2 °C). The film-coating products on the metal panel were scraped with a 45-angle pencil to the coating surface and pushed away more than 6.5 mm at a uniform speed. The pencils with different hardness levels, from the softest (6B) to the hardest (6 H), were evaluated, and the hardness of the coating sample was recorded by considering the maximum hardness level at which the film did not scratch. Each sample of coating products was carried out in triplicate.

The flexibility of the coating samples on the metallic sheet was assessed by a cylindrical mandrel tester (BGD 564, Biuged Laboratory Instruments Co. Ltd., Guangzhou, China) according to ASTM D522 under room temperature (25 ± 2 °C). The coating products were bent uniformly at 180° within 15 s using three different mandrel diameters (1/2, 1/4 and 1/8 inch). The measurement was recorded as a passed/failed test to determine the smallest mandrel size at which the coating sample can bend without cracking and/or detaching from the substrate. Each coating product was repeated in triplicate.

The gel content related to the crosslinking density of the epoxy/rubber thermosets was measured by Soxhlet extraction. The sample was extracted with excessive acetone at 60 °C for 6 h. The gel fractions were dried using a vacuum oven to obtain a constant weight. The number of insoluble fractions was calculated as illustrated in Eq. 3. The average value and standard deviation were significantly determined.

where wo and w are the dry weight of samples before and after a Soxhlet extraction, respectively.

In addition, the soluble fraction was precipitated in cold methanol (0 ± 2 °C). The precipitated rubber, referred to as the unreacted LENR after Soxhlet extraction, was dried in a vacuum oven at 60 °C until a constant weight was obtained. The weight of each sample was measured in triplicate to acquire the mean value and standard deviation. The reacted rubber content was determined using Eq. 4.

where wi is the dry weight of rubber in the printed sample before a Soxhlet extraction, and wp is the dry weight of precipitated rubber in the sol content after Soxhlet extraction.

The uniaxial tensile properties were assessed according to ASTM D882 using a universal tensile testing machine (Instron 5566, Instron, High Wycombe, UK) with a constant 1-kN static load cell capacity. Six specimens of each sample with the dimension of 10 mm × 100 mm × 0.2 mm (Width × Length × Thickness) were measured with a 50 mm/min crosshead speed at room temperature (25 ± 2 °C). Young’s modulus, tensile strength, and elongation at the break of the cured products were achieved.

The phase morphologies of the coated samples were investigated. The waterborne epoxy resin without and with modified rubber blends was coated on the wax paper (7.5 cm × 15 cm) using a bar coater by controlling the thickness of a 50-µm coated material. The coated products were left at room temperature and dried over seven days before peeling. Subsequently, the peeled film samples were cryogenically fractured in liquid nitrogen and the samples were uniformly coated with platinum/palladium (Pt/Pd, 80/20) before measurement of the phase morphology using a scanning electron microscope (SEM) (Hitachi SU 8000, Hitachi, Ibaraki, Japan).

The thermal decomposition temperature of rubbers and epoxy/rubber thermosets was evaluated by a thermogravimetric analyzer (TGA) (Mettler Q500, TA Instrument, New Castle, DE, USA). The samples were carried out in the temperature profile between 40 and 600 °C with a heating rate of 10 °C/min under a nitrogen atmosphere.

The glass transition temperature (Tg) of the rubbers and the cured coating material was investigated with a differential scanning calorimeter (DSC) (Q200-RCS90, TA Instrument, New Castle, DE, USA). the samples were performed at a heating rate of 10 °C/min from − 70 to 180 °C.

LENR was synthesized from NR latex through a two-step chemical modification as demonstrated in Fig. 1. The epoxidation was carried out using in-situ performic acid generated from the reaction of formic acid and hydrogen peroxide, resulting in partial epoxidation on the NR molecule forming epoxidized NR (ENR). Further addition of hydrogen peroxide in the ENR latex leads to oxidative degradation reaction, receiving low molecular weight ENR (LENR). Different molecular weights of LENR were prepared and defined as LENR-x, where x is the molecular weight of the modified rubber.

The chemical structure of NR, and its modified forms; ENR and LENR was assessed by the attenuated total reflection accessory equipped with Fourier transform infrared (ATR-FTIR) and proton nuclear magnetic resonance (1H-NMR) spectroscopies. Figure 2 demonstrates the ATR-FTIR spectra of the modified NRs compared to the spectrum of NR as a raw material. The unique characteristic vibrational peaks of NR were observed at 836 cm–1 and 1664 cm–1, belonging to the unsaturated parts of polyisoprene, 1376 cm–1 and 1447 cm–1, assigning to the C-H bending and the C-H stretching was found at 2800–3000 cm[–122,23. After the epoxidation reaction, the obtained ENR revealed similar characteristic signals as the NR with the new absorption peaks at 870 cm–1 and 1251 cm–1, corresponding to the C-O-C stretching with the asymmetric and symmetric modes of the oxirane ring, respectively27. Further addition of hydrogen peroxide (the second step of modification) to the ENR latex affected the cleavage of the molecular chain leading to lower molecular weight of the rubber in the latex stage, and also complementary epoxidation reaction was detected, exhibiting an increment in the peak intensity of C-O-C stretching vibration of the epoxide ring and a decline in peak intensity of the double bond of the isoprene units in the obtained LENR. Three types of LENR were prepared using different amounts of hydrogen peroxide added in the second step of the reaction, i.e. LENR-70k, LENR-100k and LENR-140k, as seen in Table 2.

ATR-FTIR spectra of NR, ENR and LENRs at various ranges of wavenumber: (a) 4000 –400 cm–1 and (b) 950 –765 cm–1.

Subsequently, the 1H-NMR technique was utilized to assert the intricate structure and determination of the epoxide content in the modified NRs. Figure 3 shows the spectrum of NR demonstrating the three unique signals at 5.14 ppm (a), 2.04 ppm (a') and 1.68 ppm (a"), assigning to the methine proton, methylene proton and methyl proton of the isoprene unit, respectively. The modified NR (ENR and LENRs) showed new signals associating with the methine proton and methyl proton of the epoxide functional group at 2.70 ppm (b) and 1.28 ppm (b'), respectively22,23. Compared to ENR, LENR samples revealed a higher signal intensity of epoxide protons (b and b’) which is consistent with the ATR-FTIR result. According to the comparative study of epoxide content determination of epoxidized natural rubber (ENR) carried out by Burfield and co-workers28, the titration method could be applicable only at low epoxide content (less than 15 mol%) as at high epoxide content, cyclization side reaction between adjacent epoxy groups could occur. 1H and 13C NMR methods gave reasonable precision at 20–75 mol%, and the DSC technique to determine the Tg of the ENR could provide the highest precision but requires an independent calibration curve. Therefore, the determination of the epoxy content of the LENR by 1H NMR method was used in our study. The epoxide content of ENR and LENRs was evaluated using the integration of signal area following Eq. 2 (shown in the experimental section), and the calculated results are summarized in Table 2. The prepared ENR under the given condition possessed an epoxide content of approximately 16%. After the oxidative chain cleavage, all LENRs revealed an increment of epoxide content by approximately 10% for all concentrations of hydrogen peroxide (0.5, 1.0, and 1.5 mol). It could elucidate that the added hydrogen peroxide in the second step may promote the epoxidation reaction through the reaction with formic acid residue, generating in situ performic acid in the reaction mixture, resulting in an increment of epoxide content.

1H-NMR spectra of NR, ENR and LENRs.

The molecular weight of the NR and modified NRs was measured using gel permeation chromatography (GPC). The distribution curves of the molecular weight of rubbers were shown in Figure S2 in Supplementary information, and the relevant values, including the number average molecular weight (Mn), the weight average molecular weight (Mw), and the polydispersity index (PDI), are listed in Table 2. The unmodified NR had the Mw value of approximately 1,100,000 g/mol with a PDI of about 1.4. After the epoxidation reaction, the obtained ENR possessed the Mw of around 400,000 g/mol with a PDI of about 1.7. The second step reaction resulted in the Mw of LENR of approximately 140,000 g/mol (LENR-140k), 100,000 g/mol (LENR-100k), and 70,000 g/mol (LENR-70k) with the PDI of about 1.8–1.9 for the use of hydrogen peroxide concentration of 0.5, 1.0 and 1.5 mol, respectively. As a result, it could be attributed that the hydrogen peroxide acted as an oxidizing agent leading to the chain breaking through the double bond of isoprene units29. As for the thermal properties of rubbers (see Table 2), the glass transition temperature (Tg) measured by DSC analysis showed a significant shift from approximately − 67 °C of unmodified NR toward a higher temperature zone by around 16 °C of ENR and 24 °C of LENRs. This phenomenon might be due to the stiffness of the epoxide ring on the modified NRs. Furthermore, the 5% weight loss decomposition temperature (T5%) investigated by TGA analysis revealed an improved initial decomposition temperature of LENR by about 7 °C compared to the unmodified NR.

The curing aspect has a vital impact on the change of mechanical, morphological and thermal properties of the thermoset epoxy system30. In this study, the LENR latex. The amine/epoxy stoichiometric ratio determined from the AHEW of Anquamine 721 and EEW of epoxy resin was 1.62, which is the typical non-stoichiometric amine/epoxy ratio with epoxy-rich formation, providing the cured material with high hardness and chemical resistance, but might lead to incomplete curing. The addition of the LENR slightly raised the EEW of the epoxy part, leading to a slight decrease in the amine/epoxy stoichiometric ratio (1.5–1.60) (Table S1 in Supplementary information). The LENR was utilized as a modifier to improve the flexibility of the epoxy resin, the same amount of the amine was used as in the control sample and is still in a non-stoichiometric amine/epoxy ratio with epoxy-rich formation. The blend formulation using three types of LENR latex (LENR-70k, LENR-100k and LENR-140k) is shown in Table 1 and the ratio of the epoxy-amine control sample is according to the vendor’s information. The kinetic curing profiles of epoxy without and with modified rubbers were assessed from the epoxide conversion using the ATR-FTIR analysis. Figure 4 shows the ATR-FTIR spectra of the uncured and cured epoxy/50LENR-100k in one day curing time. Compared to the uncured sample, the spectrum of the cured product exhibited a decrease in three noticeable absorption peaks at 915 cm–1 (C-O-C stretching of epoxide group), 1607–1641 cm–1 (N-H stretching of primary amine) and 3380 cm–1 (allotting the overlying of O-H and N-H stretching of the evaporated water and the primary amine hardener, respectively) without no change in the double bond of the aromatic ring of benzene (835 cm–1)31,32. On the other hand, there was a notable escalation of the C-H stretching (2850–2970 cm–1) and the C-H bending (1033, 1247, 1456 and 1509 cm–1) of the cured sample. These phenomena could be attributed to the crosslinking reaction between the epoxy and the active amine hardener presented in the system through the oxirane ring opening mechanism31.

ATR-FTIR spectra of uncured and cured epoxy/50LENR-100k at one day of curing time.

The kinetic profiles of the thermosetting materials evaluated from epoxide conversion are illustrated in Fig. 5. As a result, all waterborne epoxy formulations dried film showed a rapid increase in epoxide conversion, with more than 60% within 12 h and over 70% within one day. The neat epoxy revealed the maximum conversion at approximately 85%, which could be ascribed to the final conversion of the given formulation at room temperature curing system. Generally, the low viscous liquid epoxy resin will be transformed rapidly to a highly viscous form during the crosslinking reaction; subsequently, it becomes a glassy material at the gelation and vitrification of the reactive mixture, indicating the formation of the crosslinked molecular network with broadly distributed chains and complex architectures33. The addition of modified NRs (10 wt% to 50 wt%) as a toughening agent in the waterborne epoxy system showed a gradual decrement in the final epoxide conversion from ~ 85% (neat epoxy) downward to the minimum conversion of ~ 68% by adding 50 wt% LENR. This phenomenon could be due to the long molecular chains of LENR which might promote the viscosity of the epoxy system during the crosslinking reaction process to form a three-dimensional network, resulting in the limitation of segmental chain movement, thereby reducing the conversion of the thermosetting material32. Another reason is the formation of a modified rubber-epoxy network that might hinder the further formation of the crosslinking structure. For comparison of the molecular weight of modified rubbers at the same rubber content, it was found that using LENR-70k resulted in a higher conversion rate, compared to the LENR-100k and LENR-140k. The shorter molecular chain of LENR-70k may have less impact on the viscosity of the epoxy curing system, compared to the others.

Plots of epoxide conversion of epoxy/LENR blends with various LENR contents measured by ATR-FTIR analysis: (a) epoxy/LENR-70k series, (b) epoxy/LENR-100k series and (c) epoxy/LENR-140k series.

Before applying the milky-like waterborne epoxy resin/LENR blend on the tin substrate, the viscosity profiles of the epoxy/LENR blends with various LENR contents were investigated (Figure S3 in Supplementary information). As a result, the pot life of each sample, considered at the time interval with a sharp increase of viscosity to quadruple could be obtained. The prepared waterborne epoxy resin/rubber blend system showed a stable viscosity of around 60 min, indicating the pot life of the prepared resin in this work is between 60 and 75 min. Each sample was coated on the tin plate by controlling the thickness of 50 μm with a bar coater and leaving for the formation of the crosslinked network at room temperature with several curing times (1, 3 and 7 days). Figure 6 displays the example of the physical appearance of the dried film coating on the metal panels of the waterborne epoxy resin without modified NR compared with the waterborne epoxy resin blended with 50 wt% of LENR-100k latex for one day curing time under room temperature. As a result, the film-coating samples in both formulations could dry within one day, considering that the touching-coated samples are not sticky. The epoxy/rubber blend resin was able to form a crosslinked structure and adhere to the metal panel within one day under the conditions of this research. This result aligns with the rapid increase in the epoxide conversion as aforementioned.

Physical appearance of the cured film coated on the tin plate of (a) waterborne epoxy resin without modified NR and (b) waterborne epoxy resin blended with 50 wt% LENR-100k.

The adhesion of film coatings to metallic substrates was evaluated according to ASTM D3359 using a cross-hatch adhesion test, applying and removing a pressure-sensitive tape over a perpendicular lattice pattern made in the film. The adhesion level was reported in terms of five scale levels (0B to 5B), where 0B = over 65% failure; 1B = 35–65% failure; 2B = 25–35% failure; 3B = 5–15% failure; 4B = 0–5% failure and 5B = no failure. Table 3 shows the adhesion ability of waterborne epoxy resin without and with adding LENR at different molecular weights and rubber contents at various curing timeframes (1, 3 and 7 days). As can be seen, overall, an increment of curing time witnessed an enhancement of the adhesion from 2B to 5B adhesion level, and the addition of the prepared LENR latex in the waterborne epoxy resin could significantly promote the adhesion level of the film coating on the tin surface compared to that of the film coating without the addition of modified rubber. The neat waterborne epoxy film under the given condition revealed the adhesion ability with 2B (1 day), 3B (3 days), and 5B (7 days) levels. Surprisingly, adding modified rubber enhanced the cohesion level, which can be observed for the coating samples for one and three days of curing time. The epoxy resins containing 10 and 30 wt% of modified rubber showed better adhesion ability (3B or 4B for 1 day curing time and 5B for 3 days curing time) than the epoxy resin without the modified rubber. Therefore, it can be considered that the waterborne epoxy resin/10LENR-100k, waterborne epoxy resin/30LENR-100k, and waterborne epoxy resin/30LENR-140k blends are recommended for further application. The advantage of adding the LENR in the waterborne epoxy-based thermosetting formulation can be observed, particularly for three days of the cured film, no failure area of the waterborne epoxy resin/30LENR-140k blend coating film was detected (Figure S4 in Supplementary information). This result may be attributed to the chemical reaction of epoxy resin and the LENR molecules with amine hardener to form the crosslinking and interpenetrating network. Thus, increasing the inter-and intramolecular force, the slippage of the molecular segment was restricted, resulting in an improvement of coating cohesion on the metallic surface34. The interfacial interaction between the coated material and the metal substrate was also improved because of the reaction of the amine function and epoxide group of the resin matrix and modified rubber, generating the hydroxyl group and promoting an adhesion ability between the coating material and the substrate35. Furthermore, the presence of modified rubber could increase the energy dissipation when an external force is applied, leading probably to the plastic deformation of the epoxy matrix, which can escalate the cohesion strength of the epoxy/rubber blend films12. The proposed film formation and probable crosslinked structure of the coated epoxy/LENR blend in this work are illustrated in Fig. 7.

Proposed film formation and probable crosslinked structure of the coated epoxy/LENR blend.

The surface hardness of the coating samples of the epoxy and the epoxy/LENR blends on the tin panel was conducted according to ASTM D3363 using a pencil hardness test on different hardness scales ranging from 6B (softest) to 6 H (hardest). The maximum hardness level, which the film did not scratch, was considered the acceptable coated material’s surface hardness. Table 3 displays the pencil hardness of waterborne epoxy resin without and with the LENRs of different molecular weights and rubber contents. As a result, the scratch resistance of the coating samples by pencil hardness test increased over time intervals. Moreover, there was no significant difference in the scratch-resistant ability between the waterborne epoxy resin formulations without and with the modified rubbers at each curing time. This phenomenon could suggest that adding modified rubber latex (14% dry rubber content, DRC) in the range of 10–50 wt% did not importantly alter the surface hardness of the film coated on the substrate compared to the epoxy resin without rubber. As anticipated, considering the fracture behavior, the epoxy resin without rubber revealed a typical cohesive fracture when the F-type pencil level was scratched on the coating surface. Meanwhile, the epoxy/50LENR-100k blend revealed the plastic deformation behavior on the surface coating sample after the F-type level and then formed the cohesive fracture when the H-type level was applied (Figure S5 in Supplementary information). As a result, it could be described that the presence of modified rubber in the coating formulation could provide the film-coated material with higher elasticity than that of the epoxy resin without rubber. The amine hardener was responsible for the crosslinking reaction with the epoxide functional group of the epoxy and modified rubber molecules, resulting in plastic deformation fracture on the surface of the coating25. Subsequently, the flexibility of the coated sample on the tin substrate will be assessed in the bending test.

The elasticity of the thermosetting film coated on the tin substrate was evaluated according to ASTM D522 using a cylindrical mandrel tester with three different mandrel diameters (1/2, 1/4 and 1/8 inch). Table 3 shows the bending properties of waterborne epoxy resin without and with the addition of modified NRs on the tin sheet at seven days of curing time. As a result, the epoxy resin without rubber showed a crack fracture after bending the coated substrate with the smallest mandrel size (1/8-inch). The addition of different types of prepared LENRs in the epoxy resin system could provide the elasticity of adhesion and elongation ability of the prepared coating, revealing the bending ability with a 1/8-inch mandrel size without the crack fracture or detachment of the coated material. An example of the bending test presented in Fig. 8 shows the bending properties of the coated sample on the metal substrate. The epoxy/50LENR-100k blend shows superior bending performance with the smallest mandrel diameter (1/8 inch), indicating flexible respect of the coated sample compared to the neat epoxy with the lowest bending level of 1/4 inch. As mentioned earlier the addition of LENR to the epoxy formulation decreased the epoxide conversion, hence the possible reduction of crosslinking density was achieved, leading to the decrease of its brittleness. Moreover, elastomeric material in the brittle polymer, like thermosetting materials, could generally promote the flexibility of the material36. Subsequently, considering the effect of molecular weight and rubber content, the result found that the prepared LENR latex with molecular weight (~ 70,000 g/mol to ~ 140,000 g/mol) and rubber content (10 to 50 wt%) used in this work did not have a significant difference in the bending properties. Subsequently, the tensile properties of the thermosetting materials will be investigated to assert the toughening efficacy of the LENR as a mechanical modifier.

Bending test of (a) waterborne epoxy resin without modified rubber and (b) waterborne epoxy resin/50LENR-100k blend at seven days of curing time.

The cured thin films of epoxy and epoxy/rubber blend samples at seven days of room temperature curing were investigated according to ASTM D882. The stress-strain curves of the cured epoxy/modified rubber blends demonstrated a noticeable shift from brittle to ductile behavior, setting them apart from the epoxy-based coating without rubber components (Figure S6 in Supplementary information). The resulting tensile modulus, tensile strength, and elongation at break are illustrated in Fig. 9. Under the given condition, the neat epoxy is quite rigid and brittle, revealing the highest tensile modulus (~ 2 GPa) and mechanical strength (~ 62 MPa) with a shortcoming in stretchability (~ 7%). This manner indicated the aspect of the highly crosslinked and interpenetrated network architecture37. The presence of the LENR latex (10 to 50 wt%) as a toughening agent provided the thermosetting materials with improved ductility but it experienced a gradual decline in Young’s modulus and mechanical strength. The extensibility of the epoxy/rubber blend thermosets increased from approximately 7% (neat epoxy) to around 8–9% (10 wt% LENR), 10–15% (30 wt% LENR), and 19–26% (50 wt% LENR). Young’s modulus decreased to approximately 1.3–1.5 GPa (10 wt% LENR) and further decreased to around 1.2 GPa by adding 50 wt% LENR. In terms of tensile strength, the value dropped to approximately 51–56 MPa (10 wt% LENR) and continuously decreased by almost half with increasing rubber content up to 50 wt%. These phenomena could explain that the prepared LENR as a toughening agent could improve the ductility of the thermosetting epoxy material, resulting from sufficient interfacial adhesion through physical and chemical bonding during the crosslinking reaction38. Nevertheless, a significant decrement in tensile modulus and strength may be due to the elastomeric nature of rubber, which has a lower modulus and strength than the epoxy matrix, acting as a stress concentrator in the epoxy matrix23. Furthermore, another possible explanation could be that the long molecular structure of modified NR might increase the viscosity of the epoxy/rubber blend system. This, in turn, could hinder the formation of the polymer network between the epoxy resin during the curing process, leading to a reduction in the crosslinking density of the materials39, consistent with the observed decrease in epoxide conversion discussed earlier. To assert the mechanical behavior, the gel content of the thermosetting epoxy/rubber blend, related to the crosslinking density of the bulk material, was also evaluated as demonstrated in Fig. 10. As a result, the addition of LENR caused a notable decrease in the gel content, dropping from approximately 91% for pure epoxy to a decrease of 2–5% with the addition of 10 wt% LENR, and further decreasing by around 13% with the addition of 50 wt% LENR (Fig. 10a). Moreover, the Soxhlet extraction of the cured samples was carried out and the reacted LENR in the cured sample could be evaluated. As a result, there was a gradual increase in the reacted rubber content from approximately 14% by adding 10 wt% LENR toward around 30% by adding the LENR up to 50 wt% (Fig. 10b). Increasing the LENR content could contribute to a higher possibility for LENR to react with amine hardener and participate in the network formation. However, the longer molecular chains of the reacted LENR may hinder the crosslinking reaction, resulting in a decline in the gel content of samples (Fig. 10a). This observation aligns with the epoxide conversion and the tensile properties of the prepared thermosets. Considering the effect of the molecular weight of LENR, LENR-100k could provide better tensile performance than other molecular weights in this work. Subsequently, the morphological aspect of the cured samples will also be observed to visualize the improved properties.

Tensile properties of the waterborne epoxy resin without and with various types and content of modified rubbers: (a) tensile modulus, (b) tensile strength and (c) elongation at break at seven days of curing time.

(a) Gel content and (b) reacted rubber content of the waterborne epoxy resin without and with various types and content of modified rubbers at seven days of curing time.

Generally, the microstructure of polymer blends and composites can be associated with mechanical performance. A well-uniform dispersion and distribution of the minor phase in the polymer matrix plays a crucial role in establishing the morphology–property relationship22,24. Consequently, the phase morphological aspect of the coating samples of the epoxy blend without and with modified NRs was observed by scanning electron microscope (SEM), as demonstrated in Figs. 11 and 12. The cryogenically fractured surface of the neat epoxy resin in Fig. 11a revealed a homogeneous and smooth grassy behavior. It showed no sign of plastic deformation, involving the typical characteristic of the brittle thermoset polymer40. The presence of the typical LENR-100k latex (10 wt. to 50 wt%) in the waterborne epoxy resin provided the coating material with heterogeneous and rough images (Fig. 11b-d), revealing the occurrence of plastic deformation on the fractured surface. Increasing rubber content showed more surface roughness and plastic deformation with spherical rubber droplets dispersed in the matrix. As a result, forming a heterogenous structure could elucidate the non-equilibrium thermodynamics between the rubber phase and polymer matrix due to the complexity of the low-diffusion ability and the polymer chain mobility during the crosslinking reaction in thermosetting material23. In addition, the modified rubber could participate in the crosslinking reaction via epoxide functional groups on the molecular chain of the rubber as a second phase, making the epoxy matrix more flexible41. Figure 12 shows SEM micrographs of the epoxy/rubber blends using the different molecular weights (~ 70,000 g/mol to ~ 140,000 g/mol) of the prepared LENR latex (LENR-70k, LENR-100k and LENR-140k) at the same rubber content (50 wt%). The phase morphology of the epoxy/rubber blends still displayed the two distinct phases feature, revealing the dispersed rubber phase and massive plastic deformation, indicating the characteristic of ductile fracture. Therefore, the phase morphological result asserted the mechanical improvement of the coating material by adding modified rubber compared to the neat epoxy resin.

SEM images of the cryo-fractured surface of the waterborne epoxy resin blends without and with LENR-100k latex at various rubber contents for seven days of curing time: (a) epoxy, (b) epoxy/10LENR-100k, (c) epoxy/30LENR-100k and (d) epoxy/50LENR-100k.

SEM images of the cryo-fractured surface of the waterborne epoxy resin blend with LENR latex having different molecular weight at the same rubber content (50 wt%) for seven days of curing time: (a) epoxy/50LENR-70k, (b) epoxy/50LENR-100k and (c) epoxy/50LENR-140k.

The glass transition behavior of the cured films of the epoxy and epoxy/rubber blends at seven days upon the room temperature curing system is illustrated in Fig. 13, and the glass transition temperature (Tg) is listed in Table 4. As a result, the DSC thermograms showed two distinct glass-transition temperatures, one at the low-temperature boundaries (–20 °C to − 5 °C) and the other at high-temperature boundaries (30 °C to 80 °C), associating with the Tg of the modified NR and the epoxy matrix, respectively. A gradual decrement in the Tg values of the epoxy part by adding the modified rubber contents (Fig. 13a) could reveal a significant decline in the crosslinking density of the cured epoxy/rubber blends12. This phenomenon could elucidate that an increment of the rubber part may hinder the crosslinking network density upon the epoxy-amine curing system, consistent with the noticeable drop in epoxide conversion and gel content. Additionally, the Tg of the modified rubbers, which are approximately − 43 °C as demonstrated in Table 2, shifted toward a higher temperature zone in the cured samples that may be attributed to the formation of the LENR-epoxy three-dimensional crosslinked network structure through the amine hardener system under the room temperature curing process, and hence, this outlook also impacted on a significant decline in the segmental chain motion of resin matrix, resulting in a substantial decrease in the Tg of matrix phase42. Considering the effect of the molecular weight of rubber at the same rubber content (Fig. 13b), using the longer molecular chain of rubber molecules may restrict the network formation of the epoxy/rubber blend system, resulting in a decrease in the Tg value of the cured product.

DSC thermograms of the cured waterborne epoxy resin without and with modified rubbers: (a) epoxy/LENR-100k blends with various rubber contents and (b) epoxy/50LENR blends using different molecular weights of LENR at seven days of curing time.

Furthermore, thermogravimetric analysis was carried out to investigate the thermal stability of the cured samples (Figure S7 in Supplementary information). The relevant data, including the 5% weight-loss temperature (T5%) and the maximum weight-loss rate temperature (TDTG), are summarized in Table 4. As a result, the addition of LENR-100k (10 to 50 wt%) could improve the T5% of the cured epoxy/rubber blends compared to that of the neat epoxy. The T5% values were increased from ~ 320 °C of the neat epoxy up to ~ 323 °C (10 wt% LENR-100k), ~ 329 °C (30 wt% LENR-100k), and ~ 329 °C (50 wt% LENR-100k). Concentrating on the different molecular weights of rubber, leveraging the LENR-70k showed a significant increment of the T5% by approximately 12 °C compared to the neat epoxy. Meanwhile, utilizing the longer molecular chain of rubber (LENR-140k) at the same rubber content witnessed a notable decline in T5%, but it was not lower than the epoxy without rubber. This finding could be explained by the fact that an increment of the epoxy equivalent weight (EEW) of the epoxy/rubber blend system could promote the initial degradation temperature of the thermoset materials43,44. Another probable reason is that the LENR used in this research has T5% around 361 °C (see Table 2), thereby ameliorating the initial decomposition temperature of the prepared thermosetting materials. Compared with the TDTG of the epoxy without rubber (~ 394 °C), the epoxy/rubber blend system experienced a notable decrement in the TDTG by around 12 °C. This phenomenon could probably be ascribed to a decrement in the crosslinking density of the epoxy network9, which is consistent with the result of the epoxide conversion and gel content as aforementioned. Moreover, the formation of the LENR-epoxy network structure may also impact the reduction of the thermal stability of the epoxy matrix.

The development of waterborne epoxy resin with improved ductility and coating performance was achieved by effectively using low molecular weight epoxidized natural rubber (LENR). The LENR latex of approximately 25% of epoxide contents with three molecular weights of ~ 70,000 g/mol (LENR-70k), ~ 100,000 g/mol (LENR-100k) and ~ 140,000 g/mol (LENR-140k) was synthesized through a two-step of chemical modification, including epoxidation and oxidative degradation reaction. The addition of LENR latex (10–50 wt%) to the waterborne epoxy resin provided the epoxide conversion over 70% within a curing time of one day. The pot life of the prepared epoxy/LENR blends is around 60–75 min. The coated materials on the metal surface provided the maximum adhesion ability at 5B level within three days of curing time compared to that of the epoxy without rubber (3B adhesion level at the same curing time) without the deterioration of surface hardness of the coated material. The bending test involving the flexibility of the coated material on the substrate revealed a notable improvement in the bending performance of the epoxy/modified rubber coating system, without the crack and detachment after bending with the smallest mandrel diameter (1/8 inch) compared to the neat epoxy. The tensile test of the cured sample displayed the brittle-to-ductile material by adding LENR as a mechanical modifier, revealing improved elongation ability by approximately 370% with the proper gel content compared to the neat epoxy. The phase morphological aspect of the epoxy/LENR coating showed heterogeneous and rough features, belonging to the characteristics of ductile aspect compared to the smooth and glassy fashion of the epoxy coating without rubber component. Consequently, utilizing modified NR latex as an eco-friendly mechanical modifier presents a sustainable approach to address the challenge of epoxy resin-based thermoset material in coating applications. Notably, it contributes to the use of NR, a renewable material, hence reducing petroleum-based material in the coating technology for climate change mitigation.

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The authors thank the National Research Council of Thailand for financial support (Grant no. NRCT-165/2564). The instrument facilities provided by the Department of Chemistry and Rubber Technology Research Center, Faculty of Science, Mahidol University are greatly appreciated.

This research work was funded by the National Research Council of Thailand (Grant no. NRCT-165/2564).

Department of Chemistry, Faculty of Science, Mahidol University, Rama VI Road, Payathai, Bangkok, 10400, Thailand

Wasan Tessanan, Thanchanok Ratvijitvech, Taweechai Amornsakchai & Pranee Phinyocheep

Rubber Technology Research Center, Faculty of Science, Mahidol University, Phutthamonthon Sai 4, Nakhon Pathom, 73170, Thailand

Sombat Thanawan

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W.T.: Conceptualization, methodology, validation, writing-original draft. T.R., S.T., and T.A.: Conceptualization, validation, and review. PP: Conceptualization, methodology, validation, review and editing.

Correspondence to Pranee Phinyocheep.

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Tessanan, W., Ratvijitvech, T., Thanawan, S. et al. Development of waterborne epoxy-based resin incorporated with modified natural rubber latex for coating application. Sci Rep 14, 26603 (2024). https://doi.org/10.1038/s41598-024-77990-7

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