The epoxy intermediates with three hydroxyl groups were synthesized from 2-aminoethanol (ETA) with ethylene glycol diglycidyl ether (EGDE) or resorcinol diglycidyl ether (RDGE), followed by the neutralization of the inherent tertiary amine groups. The intermediate was then reacted with isophorone diisocyanate (IPDI) to produce polyurethane prepolymer, which was further chain-extended and modified by polyethylene glycol (PEG-1000) and polyethylene glycol monomethyl ether (MPEG-1200), respectively. Finally, methyl ethyl ketone oxime (MEKO) was used to block the residual isocyanate (NCO) group, leading to the achievement of the waterborne polymeric crosslinkers containing blocked NCO moieties and active epoxy groups. The structure of the crosslinkers was characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance hydrogen spectroscopy (1H-NMR). The thermal properties of the crosslinkers were studied by TG and DTG. The mechanical properties of the cured films obtained by applying crosslinkers to electrophoretic coatings were also studied. The results indicate that the introduction of epoxy groups has a significant effect on the mechanical properties of the cured films, and the RGDE crosslinker system is the optimal selection.
Research Article
Novel Waterborne polymeric Crosslinkers Containing Blocked Isocyanate Moieties and Active Epoxy Groups for Electrophoretic Coatings
https://doi.org/10.21203/rs.3.rs-3090051/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 12 Sep, 2023
You are reading this latest preprint version
waterborne polymeric crosslinker
epoxy group
blocked isocyanate
electrophoretic
With the gradual enhancement of people's environmental awareness, the development and application of water-based coatings with low volatile organic compounds (VOC) have attracted more and more attention [1–4]. In particular, electrophoretic coatings, due to the advantages of low solvent content, small pollution, and recyclable coating process, plays a more and more important role in water-based coatings and have been widely used in automotive [5], household electrical appliances [6], and other ironware coatings. In addition, the cathodic electrophoretic coatings in the market are mainly three types: epoxy resins, polyacrylate, and polyurethane. As usual, blocked polyisocyanate crosslinker is a necessary component for the coating system to improve the adhesion, hardness, and corrosion resistance of the cured films.
Some scholars have found [7–10] that by introducing epoxy resin moieties into the polyurethane chain, the hardness and corrosion resistance of the cured films can be obviously enhanced, as well as the ensurence of toughness and adhesion. However, the brittleness of the cured films will increase with the increasing amount of epoxy resin moieties. According to the current electrophoretic process, the combination of amino resin and isocyanate crosslinkers has become the universal choice, because it can crosslink with both carboxyl groups and hydroxyl groups, leading to the formation of a dual crosslinked network structure. It is noted that formaldehyde will surely be released from the amino resins, which is certainly not environmentally friendly.
Based on environmental protection and the improvement of the comprehensive properties of cured films, we decide to introduce active epoxy groups to polyisocyanate crosslinker instead of polyurethane resin. While under the curing process, the active epoxy and deblocked isocyanate (NCO) groups will react with carboxyl and hydroxyl groups, respectively. For one thing, we can also obtain films with dual crosslinked network structures. For another, the disadvantages of amino resins and epoxy resins can both be easily avoided. To our knowledge, this design idea and the relevant crosslinkers have seldom been reported.
In this study, based on the ring-opening reaction of epoxy groups from ethylene glycol diglycidyl ether (EGDE) or resorcinol diglycidyl ether (RDGE) with primary amine from 2-aminoethanol (ETA) under low temperature and alkaline conditions, we synthesized an intermediate containing trihydroxy and double epoxy groups [11]. Then, the intermediate was used to react with isophorone diisocyanate (IPDI) to prepare polyurethane prepolymer, which was subsequently chain-extended and modified by hydrophilic monomers polyethylene glycol (PEG-1000) and polyethylene glycol monomethyl ether (MPEG-1200). Finally, methyl ethyl ketone oxime (MEKO) was used to block the residual NCO groups, resulting in the achievement of waterborne crosslinkers containing active epoxy groups and blocked NCO moieties. FT-IR and 1H-NMR were used to characterize the structure of the crosslinkers. TG and DTG were used to investigate the thermal properties of the crosslinkers. Moreover, under the variation of molecular weight and structure of the crosslinkers, a comparative study was conducted on the mechanical properties of coatings obtained by applying crosslinkers to electrophoretic processes and traditional thermal curing processes.
Materials
EGDE, epoxide number 0.7 and RDGE, epoxide number 0.7, which were obtained from Foshan Hongshuo Chemical Co., Ltd; Dipropyleneglycol dimethyl ether (DPDM), which was industrial grade, was obtained from Anhui Lixin New Materials Co. LTD; ETA and Glacial acetic acid (Ac), which were analytically pure grade, were provided by Shanghai Aladdin Biochemical Technology Co., Ltd; IPDI, which was also industrial grade, was brought from Wanhua Chemical Group Co., Ltd; PEG-1000, Mn=1000, and MPEG-1200, Mn=1200, were obtained from Jiangsu Haian Petrochemical Plant; MEKO, which was Chemically pure grade, was purchased from Shanghai Maclin Biochemical Technology Co., LTD; Water-based electrophoretic resin, hydroxyl group content 1.33 mg of KOH/g) was brought from Wuhan Collier Electrophoretic Coating Co. LTD.
Physicochemical Measurement
FT-IR spectra were recorded on a NEXUS-670 FT-IR spectrometer in the spectral range of 4000–400 cm-1. The samples were prepared as KBr disks.
1H-NMR spectra were recorded by the AVANCEAV-400 from BRUKER with (CD3) 2CO as solvent.
The curves of TG and DTG were measured by the STA8000 synchronous thermal analyzer from PerkinElmer, under N2 atmosphere, with a heating rate of 10°C/min from 30°C to 500°C.
Adding 1 g of crosslinker into 10 g deionized water、10 g hydrochloric acid solution (pH = 5) or 10 g sodium hydroxide solution (pH = 9), then observe the dispersion status after magnetic stirring for 5 min. In addition, if the crosslinkers can keep stable for 15 min under centrifugation at a speed of 4000 r/min, its storage stability period of 6 months can be considered [12].
Preparation and measurement for coating films
20g electrophoretic resin was added and dispersed in 160 g deionized water. 3 wt‰ lactic acid and 10 wt% crosslinker (relative to resin) were then added subsequently to the coating system. The electrophoretic coating films were prepared by placing the iron sheet into the compound resin for 30 s at a voltage of 25 v. In comparison, the ordinary films were also prepared by rolling the resin onto the iron sheet through a 50 µm wire rod. The above two types of coating films were cured at 140°C for 30 min. The mechanical properties of coating films, such as solvent resistance、hardness, and adhesion, were researched in detail.
Using the RJCS solvent-resistant wiping instrument from Shanghai Pushen Chemical Machinery Co., Ltd., the samples were measured at a wiping frequency of 60 r/min, in accordance with GB/T 23989 − 2009.
Using the QHQ-A portable pencil hardness tester, the films were measured in accordance with GB/T 6739 − 1966.
Using PosiTestAT-M digital adhesion tester from DeFelsko, USA, the measurement is in accordance with ISO 4624 − 2016.
Synthesis of waterborne polymeric crosslinkers
The calculated amount of 10.00 g EGDE (or RDGE) and 10.00 mL DPDM were put into a 250 mL four-necked flask equipped with nitrogen protection, condensation reflux, and mechanical agitation. The mixture was then heated to 40°C, and a solution of 1.07 g (0.0175 mol) ETA in 5.00 mL DPDM was added dropwise into the flask within 15 min, and kept stirring for an additional 30 min. Subsequently, the mixture was cooled to 10°C in the ice bath, and 1.05 g (0.0175 mol) Ac was added within 20 min through the constant pressure drip funnel, which was kept stirring for 10 min to achieve intermediates with three hydroxyl groups. A solution of 14.90 g (0.13 mol) IPDI in 10.00ml DPDM was added dropwise into the flask and the system was kept at 60 °C for 1h. When the NCO content reached to the theoretical value, adding the solution of 5.00 g (0.005 mol) PEG-1000 and 36.00 g (0.03 mol) MPEG-1200 in 30.00 mL DPDM to the flask and keeping the temperature for 3 h. After that, 3.50 g (0.04 mol) MEKO and 10.00 mL DPDM were successively added to the system. Cooling the system to room temperature when the NCO content of the system is measured to be 0. Thus, we obtained two waterborne polymeric crosslinkers containing both epoxy groups and blocked isocyanate E-WPUC and R-WPUC, derived from the synthesis route of EGDE and RDGE, respectively.
E-WPUC: FT-IR(KBr, cm− 1): 3340༈NH of -NH-CO-༉, 2940༈CH of CH2, CH3༉, 1733༈CO of -NH-CO-༉, 1230༈-C2H3O༉;1H-NMR༈༈CD3༉2CO, 400MHz༉༚δ = 4.14–4.08༈2H, NH༉, 3.77–3.70༈1H, O-CH༉, 3.51–3.33༈2H, N-CH、N-CH2༉, 2.56–2.51༈2H, O-CH2༉, 1.13–0.87༈10H, CH2、CH3༉.
R-WPUC: FT-IR(KBr, cm− 1): 3340༈NH of -NH-CO-༉, 2940༈CH of CH2, CH3༉, 1733༈CO of -NH-CO-༉, 1230༈-C2H3O༉;1H-NMR༈(CD3)2CO, 400MHz༉༚δ = 7.20–6.49༈7H, C6H6༉, 4.32–4.25༈1H, O-CH༉, 4.19–4.06༈2H, NH༉, 3.51–3.38༈2H, N-CH、N-CH2༉, 2.84–2.79༈2H, O-CH2༉, 1.15–0.86༈10H, CH2、CH3༉.
According to the above reaction process, the isocyanate indexes of polymerization for both kinds of crosslinkers are shown in Table 1.
$${R}_{1}=\frac{{\left[NCO\right]}_{IPDI}}{{\left[OH\right]}_{Intermediate}};{R}_{2}=\frac{{\left[NCO\right]}_{Prepolymer}}{{\left[OH\right]}_{PEG}+{\left[OH\right]}_{MPEG}}$$
The prepared crosslinkers are named as E-WPUC-1.8, E-WPUC-2.0, E-WPUC-2.2, E-WPUC-2.4, E-WPUC-2.6 and R-WPUC-1.8, R-WPUC-2.0, R-WPUC-2.2, R-WPUC-2.4, R-WPUC-2.6.
Synthesis
The synthesis route of the waterborne polymeric crosslinkers was shown in Scheme 1. Firstly, through the ring-opening reaction, EGDE or RDGE was used to react with ETA to achieve intermediates with trihydroxy functionality and two active epoxy groups. Secondly, the intermediates were reacted with IPDI to obtain NCO-terminated branched prepolymers, which were then hydrophilic modified, and chain-extended by PEG and MPEG. Finally, the residual NCO groups were blocked by MEKO.
The FT-IR spectra of E-WPUC-2.2 and R-WPUC-2.2 were shown in Fig. 1. The typical stretching vibration peaks of polyurethane were observed at 3400 cm− 1[ν(NH)], 2940 cm− 1[ν(CH3) and ν(CH2)], and 1700 cm− 1 [ν(C = O)] in both spectra. The peaks of the aromatic ring from RDGE moieties can be observed at around 1650 cm− 1[ν(C6H6)], and the characteristic absorption peak of epoxy groups can be observed at around 1100 cm− 1 [ν(-CHOCH-]. Moreover, no absorption peaks were found around 2260 cm− 1, which indicates all NCO groups have been completely blocked.
The structures of crosslinkers were further confirmed by 1H-NMR spectra, as shown in Fig. 2. The peaks located between 7.20–6.49 ppm were contributed by the proton of RDGE moieties. The peaks located at 4.10 ppm can be attributed to N-H from urethane moieties. The peaks located at 3.80 ppm and 2.83 ppm were assigned to the protons of the epoxy groups. The peaks located at 2.69 ppm were the characteristic methylene peak derived from IPDI moieties. As the results mentioned above, the waterborne crosslinkers have been successfully synthesized.
Water dispersion, pH stability, and storage stability
| Crosslinkers | pH Stability | Storage Stability /Months | ||
|---|---|---|---|---|
| Distilled water | HCl solution | NaOH solution | ||
| E-WPUC-1.8 | + | + | + | >6 |
| E-WPUC-2.0 | + | + | + | >6 |
| E-WPUC-2.2 | + | + | - | >6 |
| E-WPUC-2.4 | + | + | - | <6 |
| E-WPUC-2.6 | + | + | ○ | <6 |
| R-WPUC-1.8 | + | + | + | >6 |
| R-WPUC-2.0 | + | + | - | >6 |
| R-WPUC-2.2 | + | + | - | >6 |
| R-WPUC-2.4 | - | - | ○ | <6 |
| R-WPUC-2.6 | - | - | ○ | <6 |
+ : Bluish transparent; -: Bluish white semitranslucent; ○: Flocculent precipitation
As shown in Table 2, the storage stability of both kinds of crosslinkers has a critical R2 value of 2.2. While below the critical value, the storage period can be presumed to be longer than 6 months. On the contrary, the storage stability of both crosslinkers will be deteriorated. This result can be ascribed to the decrease of the polar hydrophilic unit of crosslinkers, leading to the relatively poor compatibility of the whole system. From Table 2, compared with the relatively stable water dispersibility of E-WPUC, the dispersibility of R-WPUC will gradually decrease with increasing R2 value, which can be attributed to the effect of the hydrophobicity of its rigid benzene ring. As for acid-base stability, the dispersion ability of all crosslinkers was not affected in the hydrochloric acid solution (pH = 5). However, under the alkaline condition (pH = 9), with the increase of R2 value, the water dispersion of the two crosslinkers gradually changed from blue transparent liquid to flocculent precipitation, which can be attributed to the deterioration of active epoxy groups of crosslinkers. While in alkaline conditions, the active epoxy groups can be easily induced to generate ring-opening polymerization, resulting in the increase of the molecular weight of crosslinkers. Certainly, the higher the content of epoxy groups in crosslinkers, the poorer their dispersibility in alkaline solution. Instead, the ring-opening condition of epoxy groups is very strict in acidic solution.
Thermal stability
The thermal behavior of the crosslinkers was studied through TG and DTG curves, as shown in Fig. 3. From Fig. 3, both crosslinkers can reach the first weight loss peak at 96°C, which can be attributed to the deblocking of the blocked NCO groups. Due to the decomposition of the hard segment in crosslinkers, the second main mass of E-WPUC-2.2 and R-WPUC-2.2 began to decrease slowly at 196°C and 231°C, respectively. The soft segment of E-WPUC-2.2 began to decompose at 279°C and reached the maximum decomposition rate at 315°C. In contrast, the decomposition temperature of R-WPUC-2.2 raised to 400°C and reached the maximum decomposition rate at 455°C [11]. This difference is mainly attributed to the contribution of the rigid benzene ring moieties in R-WPUC. Therefore, the thermal analysis study showed that the active NCO groups in crosslinkers had been completely blocked, and it was also confirmed that both crosslinkers can be deblocked and form effective crosslinked network with resins at a temperature higher than 96°C。
Properties of cured films
Curing mechanism
Scheme 2 Mechanism Diagram for curing process between crosslinkers and resins
Solvent resistance
Figure 4 shows the relationship between MEK wiping resistance of the cured films and R2 value. From Fig. 4, the wiping resistance of all films gradually increases with the increase of R2 value, which is due to the relative increase in the content of blocked NCO groups and epoxy groups in the crosslinker, resulting in an increase in the crosslinking density of the cured films. Moreover, the wiping resistance of films prepared by the electrophoretic process is significantly better than that of other films, especially for those prepared by R-WPUC. Compared with the random arrangement of resin molecules in ordinary heat-cured films, the resin molecules will be arranged much more neatly under the action of an electric field, resulting in a relatively denser crosslinked structure. In addition, in contrast with the aliphatic crosslinker E-WPUC, R-WPUC contains many rigid aromatic ring structures, so the solvent resistance of the latter system is relatively better than the former.
Hardness
Figure 5 depicts the effect of changes in R2 value on the hardness of the cured films. According to Fig. 5, Compared with the films prepared by wire rod, the hardness of the films prepared by the electrophoretic process is obviously higher, which is due to the more uniform and dense arrangement of resin molecules under the action of an electric field. At 140°C, the active epoxy groups in the crosslinkers can generate an independent crosslinked network by nucleophilic addition reaction with the carboxyl groups. Meanwhile, the deblocked NCO groups and the hydroxyl groups in the resin can also form an independent crosslinked network. In addition, with the increase of R2 value, the hardness of the electrophoretic films gradually decreases, while the hardness of the wire rod films is the opposite. Under the effect of the electric field, the polymer chains will be tightly arranged along the electric field direction, which is beneficial for improving the hardness of the films. As should be pointed out here, due to the content of carboxyl groups in the resin is lower than that of hydroxyl groups, the unreacted epoxy groups, present as branched chains, will certainly weaken the intermolecular forces of the crosslinked structure, resulting in a decrease in the hardness of the films.
As for the wire rod method, the hardness of the films increases with the increase of R2 value. Due to the less compact arrangement of its polymer chains compared to that of the electrophoretic films, the reaction between epoxy groups and carboxyl groups, as well as the deblocked NCO groups with hydroxy groups, is relatively more difficult. Therefore, as R2 value increases, the content of the crosslinkable groups will also increase relatively, which is beneficial for improving the crosslinking reaction between them, resulting in a denser density of the formed crosslinked network.
Adhesion
The relationship between R2 value and the adhesion of the cured films is shown in Fig. 6. From Fig. 6, when R2 value amounts to 2.2, the adhesion of all films reaches its maximum value. This is mainly because when R2 value is less than 2.2, the NCO groups, released from the crosslinkers, is not sufficient to fully react with the hydroxyl groups in the resin, resulting in a relatively low crosslinking density of the final films. At the same time, as R2 value continues to increase, the amount of epoxy groups and active NCO groups provided by crosslinkers will also correspondingly increase, leading to the adhesion of the films goes up gradually. On the contrary, When R2 value is higher than 2.2, the adhesion of all films decreases gradually, which is mainly due to the further increase in the content of crosslinkable groups, the crosslinking density of the films will also correspondingly increase, leading to an increase in the cohesion of the film itself, thereby weakening its adhesion to the substrate. In other words, during the crosslinking process, the molecular weight of the resins will be rapidly increased, together with structure transitions from linear to network. As a result, the viscoelasticity of polymer chains will gradually shift toward the elastic direction.
Consistent with the testing of solvent resistance and hardness, the adhesion of films prepared by the electrophoretic process, due to the relatively tighter stacking of polymer chains, is significantly better than that of films prepared by the wire rod process.
In this study, we synthesized an active intermediate containing hydroxyl and epoxy groups, and then introduced it into the polymer chain to obtain novel waterborne crosslinkers containing active epoxy and blocked NCO groups, which are expected to be used to achieve dual-crosslinked electrophoretic coating films. FT-IR and 1H-NMR confirmed the structure of crosslinkers. Researches on water dispersion and acid-base stability indicate that E-WPUC has better water dispersibility than R-WPUC, and can be stably dispersed under both neutral and acidic conditions. Thermal analysis shows that both crosslinkers can quickly crosslink and cure the resin at temperatures above 96°C. Studies on the mechanical properties of the cured films show that the cathodic electrophoretic film prepared by R-WPUC has relatively better performance. Furthermore, the adhesion of all films reaches its maximum value at an R2 value of 2.2. In a word, the waterborne polymeric crosslinker R-WPUC-2.2 might possess greatly applied potential in electrophoretic coating fields.
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