Crosslinked polyurethane-epoxy hybrid emulsion with core-shell structure.
Keywords Crosslinking, Epoxy resin, Polyurethanes emulsion
Waterborne polyurethanes (WPUs) have excellent mechanical properties and are environment-friendly. However, WPU resins have poor resistance against water and chemicals compared with the crosslinked two-pack solvent-based urethanes. This is because most commercial WPUs have ionic groups, which are hydrophilic in nature, and are linear thermoplastic polymers with little gel content, which make them easy to be dissolved. (1-5)
To enhance the performance of WPU, these polymers have been reported to be crosslinked with aziridine, (6) carbodiimide, (7) amino-formaldehyde and melamine-formaldehyde resins, (8) etc. Furthermore, a separate polymer could be incorporated to polyurethane to form a multiphase structure in the dispersions through various techniques such as blending, seeded emulsion polymerization or interpenetration network formation, etc. (4), (9-15) Epoxy resin (epoxide equivalent is higher than 230 g/eq) is a multihydroxide compound, and can introduce branchpoints to the backbone chain of polyurethane to form a reticulated crosslinked structure. Therefore, using epoxy resin to modify WPU can combine the advantages of both of these materials and results in improved film performance. (16-20)
Epoxy resin has been used to precrosslink WPU based on the reaction between the isocyanate-terminated prepolymer with the epoxy group and hydroxyl group of the epoxy resin. (19), (20) Although precrosslinking can improve the properties of emulsion and cast film, high level of precrosslinking may not be easily introduced into waterborne urethane due to the high viscosity of prepolymers or the poor coalescence of formulated emulsion. (4), (21), (22) In addition, epoxy resin has been used to precrosslink polyurethane emulsion during or after film formation. (4), (23) But the polymerization reaction proceeded smoothly, and the coagulum content was low only when the epoxy content was below 15 wt%. The emulsion obtained could be stored for only 30 days without performance changes and is not suitable for single package coating.
In this work, we explored a novel postcrosslinking method to form crosslinked polyurethane-epoxy hybrid emulsion. Epoxy resin and a blocked NCO prepolymer were mixed to avoid the high viscosity of precrosslinked polyurethane and the difficulty of emulsification. In addition, the epoxy resin and the blocked NCO prepolymer could be introduced into the interior of the latex particles during the phase inversion process due to their hydrophobicity. The cross-linking density of the hybrid resin was enhanced by adding diethylene triamine into the emulsion and reacted with block isocyanate and epoxide group at 80[degrees]C in the interior of the emulsion particles.(24), (25) Therefore, the above-obtained resin can incorporate the rigidity and adhesion property of polyurethane. Furthermore, the chain-extension process in water phase can be well controlled, and good dispersion and storage stabilities are readily obtained.
Materials and methods
2,4-Toluene diisocyanate (TDI, 80/20) was obtained from Takeda Company Limited (Osaka, Japan). Polyether diol (PED) with a molecular weight of 1000 Da (N210) was obtained from Tian-jin Petrochemical Corporation (Tian-jin, China). Dimethylol propionic acid (DMPA) was obtained from Perstorp Specialty Chemicals (Perstorp, Sweden). Epikote 1001 (hereinafter called type 1 epoxy) was obtained from Shell Corporation (Columbus, USA). 1,4-Butanediol, diethylene triamine, acetone, l-methyl-2-pyrrolidone (NMP), isopropanol, and triethylamine (TEA) having purity of 99.5% were purchased from Guangzhou Guanghua Company Limited (Guangzhou, China). The reagents were used as received in aqueous polyurethane dispersion synthesis.
A 500-[cm.sup.3] four-neck round-bottom flask equipped with a mechanical stirrer, thermometer, and nitrogen gas inlet was used as the reactor. The reaction was carried out in a water bath at constant temperature. The synthesis procedure is listed below.
Preparation of unmodified polyurethane dispersion
Forty-six gram TDI, 26.2 g PED, 8.1 g 1,4-butanediol, and 7 g DMPA were placed into the flask. NMP was added as the solvent. The mixture was stirred and reacted at 70[degrees]C for 3 h. Subsequently, 3 g isopropanol, blocking agent, was added to block a part of NCO groups until the amount of residual NCO content reached a theoretical end point, as calculated when all hydroxyl groups had reacted with isocyanate groups. The NCO content of the prepolymer was thus obtained. The polyurethane anionomer was obtained by cooling the prepolymer to 40[degrees]C, and triethylamine was added to neutralize the COOH groups of DMPA. The polyurethane anionomer was then dispersed in water with vigorous agitation, and the polymer chain was extended by reacting with 0.5-1.0 g ethylene diamine. The unmodified aqueous polyurethane dispersion was thus obtained.
Preparation of epoxy resin-modified polyurethane dispersion
PRECROSSLINKING: Forty-six gram TDI, 27.2 g PED, 8.1 g 1,4-butanediol, and 7 g DMPA were placed into the flask. NMP was added as the solvent. The mixture was stirred and reacted at 70[degrees]C for 3 h or until the amount of residual NCO content reached the theoretical end point. The solution of 10-20 g type 1 epoxy dissolved in NMP was added and copolymerized at 60-70[degrees]C for 2-3 h until the amount of residual NCO content reached the theoretical end point. Subsequently, 3 g isopropanol was added to block part of NCO to obtain the NCO-terminated polyurethane prepolymer containing ionic groups. After neutralizing with triethylamine, the mixture was emulsified in water with vigorous agitation. Chain extension was induced by immediately adding 0.5-1.0 g primary diamines to the dispersion to react with NCO group. An aqueous dispersion of polyurethane ionomer was thus obtained.
POSTCROSSLINKING: Forty-six gram TDI, 27.2 g PED, 8.1 g 1,4-butanediol, and 7 g DMPA were placed into the flask. NMP was added as the solvent. The mixture was stirred and reacted at 70[degrees]C for 3 h or until the amount of residual NCO content reached a theoretical end point. Ten gram isopropanol was added to block all NCO to obtain NCO-blocked prepolymer. The reaction temperature was then decreased to 30[degrees]C, and the solution of 10-20 g type 1 epoxy dissolved in NMP was added to the prepolymer and stirred. After neutralizing with 4.5 g triethylamine, the mixture was emulsified in water with vigorous agitation. Finally, reaction temperature of emulsion was raised to 80[degrees]C, and 7.66 g diethylene triamine was added to the emulsion and reacted for 2 h. The crosslinked polyurethane-epoxy hybrid emulsion was thus obtained (Fig. 1).
[FIGURE 1 OMITTED]
CHARACTERIZATION: The Fourier-transformed infrared spectrometer (FTIR) (VECTOR 33, Bruker Co., Germany) was carried out to determine the chemical structure of the sample. The KBr pellet technique was used to prepare the powder samples for FTIR studies, and the average of three scans for each sample was taken for the peak identification. Different scanning calorimetry (DSC) measurements were performed on STA 449C Jupiter DSC thermal analyser (Netzsch Co., Selb, Germany). The experiments were carried out at a heating rate of 10[degrees]C/min under [N.sub.2] atmosphere in the temperature range of -50 to 200[degrees]C. Transmission electron microscopy measurements were performed on Tecnai 10 (Philips Co., Holland) microscope at an accelerating voltage of 200 kV.
Tensile strength and elongation at break were conducted at ambient temperature on Instron 1185 universal testing instrument. The film of postcrosslinking polyurethane was prepared by casting on a release paper, followed by drying at room temperature for 3 h, and samples were then aged for 1 week before testing. The film thicknesses were 0.5-1.00 mm. For each film, two specimens were tested and average value reported.
To investigate the water/toluene/acetone resistance, polyurethane films were immersed in water/toluene/acetone for 24 h at room temperature. The swelling ratio was calculated as follows:
Swelling ratio (%) = ([W.sub.1] - [W.sub.0]/[W.sub.0],
where [W.sub.0] and [W.sub.1] represent the weight of dried and swollen films, respectively.
Results and discussion
Analysis of the postcrosslinking reactions
The curing reaction of the epoxy group, NCO, and amine chain extender during the chain-extension process was analyzed by IR spectroscopy. In general, the free N-H stretching is at 3447-3600 [cm.sup.-1]. (26) However, N-H stretching of polyurethane was shifted to 3294 [cm.sup.-1] in Fig. 2 for spectra of emulsion before chain extension by amine. The reason is that the N-H group and the carboxy group (C=O) of urethane has generated a spot of hydrogen bonds in our experiment, (27) which resulted in the absorption peak of N-H stretching shifted to low value. The band at 1745 [cm.sup.-1] was C=O stretching of the urethane group, and the strong band at 1223-1267 [cm.sup.-1] corresponded to the stretching vibrations of C=O combined with NH in all the spectra. (28) The band at 1607-1606 [cm.sup.-1] was benzene ring, the bands at 1533-1539 [cm.sup.-1] correspond to carbamate, and the band at 2940-2926 and 2861-2853 [cm.sup.-1] was asymmetric and symmetric C-H stretching of [CH.sub.2], respectively. The absorption peaks of NCO at 2277 [cm.sup.-1] and epoxide groups at 917 [cm.sup.-1] also appeared in Fig. 2, which revealed that epoxy resin maintained its own structure characteristics, and small part of free isocyanate (NCO) groups were generated because the sealant volatilized and the thermodynamic balance of blocking reaction moved to reserve reaction during the film forming. The absorption peaks of epoxy group and free isocyanate group at 917 and 2277 [cm.sup.-1] vanished after chain extension in Fig. 3, which indicated that a chain extender reaction occured between the isocyanate group, epoxide groups, and amine chain extender. (29) These results show that the free isocyanate (NCO) groups were generated at elevated temperature, and then, the free NCO groups could easily be reacted with epoxide group and chain extender. (30)
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Thermal study by DSC
Differential scanning calorimetry was used to analyze the glass transitions of cast film and emulsion before chain-extension reaction for PUPE1 and PUPE2 (content of type 1 epoxy at 10 and 20 wt%, respectively) and after chain extension reaction for PUPE3 and PUPE4 (content of type 1 epoxy at 10 and 20 wt%, respectively). The films were formed at room temperature.
There were two endothermic peaks at about 100 and 150[degrees]C in Figs. 4 and 5, which represented the epoxide group curing and the deblocking of isocyanate, respectively. (31), (32) Usually the temperature of block isocyanate blocked is about 166[degrees]C when blocked agent is isopropanol. In our experiments, triethylamine that was used for neutralization could catalyze the blocked isocyanate dissociation, and thus the temperature of blocked isocyanate dissociation decreased.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
In the DSC studies, the cast film of PUE4 curing was also carried out at 180[degrees]C for 2 h. The result is shown in Fig. 8. After eliminating the effect of baseline shifting, the DSC thermogram shape in Fig. 8 is similar to that in Figs. 6 and 7. This could be explained by the internal crosslinking reaction during the chain-extension process, as confirmed by the IR spectra.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
Effect of synthetic process
Due to the high reactivity between the epoxy group and the active hydrogen atom, (23) the synthetic route is the most critical factor for obtaining a stable and homogeneous emulsion in our research. Figure 9 and Table 1, respectively, show the TEM images and the performance of the crosslinked emulsion and cast film modified by different synthetic process with the molar ratio of NCO/OH at 1.4:1 and mass content of type 1 epoxy at 10%.
It can be seen from Fig. 9 that the structure of the polyurethane-epoxy emulsion prepared by precrosslinking is similar to that of nonmodified polyurethane emulsion and had no core/shell structure. In contrast, the polyurethane-epoxy hybrid emulsion prepared with postcrosslinking had clear core/shell structure, with the hydrophilic chain with ionic group forming the outer layer of the emulsion particle.
[FIGURE 9 OMITTED]
Table 1 shows that WPU could be precrosslinked or postcrosslinked to improve the mechanical and solvent properties. However, the stability of precrosslinking sample was poor and the precipitation appeared after 3 months. This may be because the epoxy resin was grafted in the molecular chain of polyurethane by precrosslinking and the wrapping was incomplete. During the process of chain extension with diethylene triamine, the reaction taking place on the outer shell of the particle may reduce the hydrophilicity of the emulsion. Furthermore, crosslinking may occur between the particles. Using the postcrosslinking synthetic process, the epoxy resin was inserted into the core of latex droplets and was encapsulated tightly by the hydrophobic chain of polyurethane after the phase inversion. As the amine chain extenders diffuse in the particles, crosslinking also occurred in the core of the particle. Therefore, the stability of the emulsion obtained was very good. The emulsion crosslinking would be postcrosslinked in the later part of this work.
Table 1: Effect of synthesis process on the performance of emulsion and its film Postcrosslinking Precrosslinking Unmodified polyurethane emulsion Viscosity of 8102 20056 7948 prepolymer before phase inversion (25[degrees]C) (cp) Particle size 172.5 195.6 81.9 (nm) Storage stability >6 months Deposition after >6 months of emulsion 3 months Water absorption 12.6 9.7 34.8 rate of film (%) Toluene 46.8 46.2 83 absorption rate of film (%) Tensile strength 19.6 25.3 8.9 of film (MPa) Elongation rate 350 290 640 at break of film (%)
Effect of chain-extension reaction temperature
Diethylene triamine with the molar ratio of amine and blocked isocyanate at 1:1 was added to the emulsion. The emulsion was stirred and the temperature was raised. Diethylene triamine entered the emulsion particle interior, and the crosslinking reaction occurred to extend the molecular chain. The results on the relationship between crosslinking degree and solidification time under different reaction temperatures are shown in Fig. 10. The reaction in the emulsion particle interior was essentially deblocking reaction of blocked isocyanate and epoxy resin solidification reaction. The deblocking reaction is usually thermodynamically controlled, and the thermodynamic balance of the reaction shifted toward dissociation when amine chain extender reacted with a small amount of isocyanate group. Thus, the reaction temperature was greatly lower than the usual deblock temperature of blocked isocyanate.
[FIGURE 10 OMITTED]
It also can be seen that the crosslinking degree increased abruptly during the reaction process for all of reaction temperature from 70 to 90[degrees]C in our experiment, and it is in favor to increase the crosslinking degree at the higher reaction temperature. However, if the reaction temperature was higher than 80[degrees]C, condensation of the emulsion particle readily occurred and resulted in gel or precipitation. The optimal reaction temperature was found to be around 80[degrees]C in our experiments.
Effect of the epoxy content
The effect of epoxy content on the properties of the crosslinked emulsion and its cast film is shown in Table 2. It can be seen that increasing the epoxy content reduced the film elongation at break and enhanced the tensile strength and water resistance of the film. The effect was more significant with the epoxy resin. Since the crosslinking density of the films increased with an increase in epoxy content, the ratio of the rigid segment in resin was enhanced. Furthermore, good stability of the emulsion can be obtained even if the epoxy mass content exceeded 10 wt%, as long as the carboxy content was increased accordingly. Thus, a high epoxy content (up to 20 wt%) could be used to improve the mechanical properties.
Table 2: Effect of the epoxy content on properties of the emulsion and its film prepared with postcrosslinking Type 1 epoxy (%) 5 10 15 20 COOH (%) 2.3 2.5 2.7 2.9 Appearance Translucency Translucency White White with blue with blue emulsion emulsion Particle size 83.1 93.1 153.3 172.5 (nm) Pencil hardness 1B 1H 1H 2H Tensile strength 17.1 19.6 31.8 33.2 (MPa) Elongation rate 520 350 330 320 at break (%) Water absorption 15.3 12.6 10.2 8.1 rate (%) Storage stability 6 6 6 6 (months)
Effect of amine chain extender
The effects of different types of amine chain extender, such as ethylene diamine and diethylene triamine, on the performance of the crosslinked emulsion and cast film were studied. The results are shown in Tables 3 and 4. In this research, isopropanol was used as the blocking agent. The molar ratio of TDI/OH was 1.4:1, and the DMPA and type 1 epoxy content was 7 and 20 wt%, respectively.
It can be seen from Table 3 that the appearance and stability of the crosslinked emulsion prepared with ethylene diamine and diethylene triamine were similar. The stability of the crosslinked emulsion was good with molar ratio of NH/NCO less than 1:1, and decreased with molar ratio of NH/NCO at 1.2:1. From Table 4 it is clear that the mechanical strength of the film prepared with diethylene triamine was higher than that prepared with ethylene diamine. The film prepared with diethylene triamine had high tensile strength and hardness and low elongation rate at break. This may be explained by the fact that diethylene triamine with multifunctionality can react with prepolymer to form crosslinked structure, and thus the molecular mass increased and the performance of the film was improved. The molar ratio of NH/NCO at 1:1 led to maximum mechanical strength of the film prepared with diethylene triamine.
Table 3: Influences of different amine chain extenders on the performance of the emulsion and film Species of amine NH/NCO Particle Storage Mechanical chain extender (molar size stability stability (3000 ratio) (nm) rpm) Ethylene diamine 0.8 110.7 >6 months >30 min 1.0 136.9 >6 months >30 min 1.2 152.1 >6 months but >30 min becoming faint yellow Diethylene 0.8 157.3 >6 months >30 min triamine 1.0 172.5 >6 months >30 min 1.2 182.4 >6 months but A little becoming faint deposition after yellow 30 min Triethanolamine 0.8 184.7 <1 month Delamination after 5 min 1.0 195.8 <1 month Delamination after 5 min 1.2 209.3 <1 month Delamination after 5 min
The results of swelling rate of films in water, toluene, and acetone are shown in Table 4. It can be seen that the water resistance and solvent resistance of film prepared with diethylene triamine were better than those prepared with ethylene diamine. This may be explained by the high crosslinked degree and intermolecular cohesive energy of polymer macro-molecules chain after the extension reaction with diethylene triamine. Starting at a low molar ratio of NH/NCO, film performance was improved with increased molar ratio of NH/NCO. But the film performance was poor when the molar ratio of NH/NCO was increased to 1:1.2. This might be explained by the decreased crosslinking degree and the presence of excessive hydrophilic amine chain extender.
Table 4: Influences of different chain extenders on the performances of postcrosslinking polyurethane-epoxy hybrid film Species of amine NH/NCO Tensile Elongation rate Pencil chain extender (molar strength at break (%) hardness ratio) (MPa) Ethylene diamine 0.8 15 340 HB 1.0 21 430 H 1.2 15 350 HB Diethylene 0.8 18 260 HB triamine 1.0 33.2 320 2H 1.2 19 270 H Species of amine Water Toluene Acetone chain extender absorption rate absorption rate absorption rate (%) (%) (%) Ethylene diamine 30 52 143 15 41 85 28 53 120 Diethylene 18 35 84 triamine 8.1 27 55 16 37 86
Crosslinked urethane emulsion could be precrosslinked or postcrosslinked by an epoxy. The postcrosslinking method could improve the film performance and the emulsion stability, and did not result in increased viscosity of the prepolymer before phase inversion because the epoxy resin did not react with prepolymer. The epoxy resin was inserted into the core of latex droplets and was encapsulated tightly by the hydrophobic chain of polyurethane after phase inversion; the crosslinking mostly occured in the interior of the particles. The emulsion had good stability, and the mass content of the epoxy resin could be increased to 20%.
In postcrosslinking, the proportion of hard chain link in polymer chain was increased, thus the modulus, the stress at same elongation rate, and the water resistance of film increased with increase in epoxy resin mass content.
The type of amine chain extender significantly affected the stability of emulsion. The hybrid emulsion with the chain extended by triethanolamine had poor stability, and the samples with the chain extended by ethylenediamine or diethylene triamine could be stored for at least 6 months without any apparent performance change. Furthermore, because diethylene triamine was a multifunctional compound, the mechanical strength and chemical resistance of the film were enhanced, and the elongation at break was decreased for the polyurethane resin with the chain extended by diethylene triamine. The molar ratio of NH/NCO at 1:1 showed the best film performance, and the optimal reaction temperature of amine extending chain was around 80[degrees]C.
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X. Wen (*), R. Mi, Y. Huang, J. Cheng, P. Pi, Z. Yang
The School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
J. Coat. Technol Res., 7 (3) 373-381, 2010