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Reversible polymer networks based on the dynamic hindered urea bond for scratch healing in automotive clearcoats
Journal Pre-proofs Full Length Article Reversible Polymer Networks Based on the Dynamic Hindered Urea Bond for Scratch Healing in Automotive Clearcoats Gi Young Kim, Sujin Sung, Minsoo P. Kim, Soon Cheon Kim, Sang-Ho Lee, Young Il Park, Seung Man Noh, In Woo Cheong, Jin Chul Kim PII: DOI: Reference:
S0169-4332(19)33362-8 https://doi.org/10.1016/j.apsusc.2019.144546 APSUSC 144546
To appear in:
Applied Surface Science
Received Date: Accepted Date:
18 October 2019 29 October 2019
Please cite this article as: G. Young Kim, S. Sung, M.P. Kim, S. Cheon Kim, S-H. Lee, Y. Il Park, S. Man Noh, I. Woo Cheong, J. Chul Kim, Reversible Polymer Networks Based on the Dynamic Hindered Urea Bond for Scratch Healing in Automotive Clearcoats, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144546
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Reversible Polymer Networks Based on the Dynamic Hindered Urea Bond for Scratch Healing in Automotive Clearcoats Gi Young Kima,b, Sujin Sunga, Minsoo P. Kimc, Soon Cheon Kima, Sang-Ho Leea, Young Il Parka, Seung Man Noha, In Woo Cheong1,*, Jin Chul Kim1,* aResearch
Center for Green Fine Chemicals, Korea Research Institute of Chemical
Technology, Ulsan 44412, Republic of Korea bSchool
of Applied Chemical Engineering, Graduate School of Advanced Integration of
Science and Technology, Kyungpook National University, Daegu 41566, Republic of Korea cSchool
of Energy and Chemical Engineering, Ulsan National Institute of Technology
(UNIST), Ulsan 44919, Republic of Korea
*Corresponding
1I.
author.
W. Cheong and J. C. Kim are the co-corresponding authors of this article.
Highlights
• Synthesis of dynamic polyacrylate urethane urea networks containing hindered urea groups. • Control of the material properties of the polymer networks using different HU adducts. • Quantitative evaluation of the scratch-healing performances using a micro-scratch tester. • Excellent mechanical and scratch-resistant properties • Better scratch-healable performance over commercial clear coats
Abstract: A series of dynamic polyacrylate urethane urea networks containing hindered urea (HU) groups (C-HU-ethylene glycol, C-HU-EG; C-HU-tetra(ethylene glycol), C-HU-TEG; CHU-poly(ethylene glycol), C-HU-PEG) were successfully synthesized for use in automotive clearcoats. The material properties of the clearcoats were controlled using different HU adducts and by adjusting their contents in the polymer networks. The indentation hardness (HIT), modulus (EIT), thermal stability (Td), and glass-transition temperature (Tg) of the polymer
networks were characterized by nano-indentation tests, dynamic mechanical analysis, differential scanning calorimetry, and thermogravimetric analysis, respectively. The scratchresistance and healing performances of the polymer networks were evaluated quantitatively using a micro-scratch tester in conjunction with optical microscopy. The results reveal that the C-HU-TEG polymer networks not only exhibited physical properties most similar to those of the commercial clearcoat (T30) but also demonstrated the best self-healing performance. The balanced chemical structure of the HU-TEG adduct between the flexible unit (TEG) and the rigid unit (IPDI, urea, and urethane bond) played an important role of increasing scratchhealing performance while maintaining the clearcoat material properties.
Keywords: Intrinsic self-healing, Dynamic hindered urea bond, Dynamic polymer network, Automotive clearcoat, Scratch resistance, Scratch healing
1. Introduction Automotive clearcoats are transparent coatings that protect the substrate surface from harsh outdoor environments for prolonged periods. In general, automotive clearcoats are poly(urethane acrylate) networks with high crosslinking density so that they exhibit good longterm mechanical stability and chemical resistance[1,2]. However, because automotive clearcoats are based on organic polymeric materials, they are still vulnerable to scratching by sharp materials such as inorganic particles and metals. To overcome this drawback, considerable research effort has been devoted to developing coatings with high scratch resistance by increasing the polymer crosslinking density and/or adding inorganic materials. However, this approach is limited in applications that require high elasticity, processability, compatibility, and/or optical transparency[3–6]. Recently, several groups have attempted to impart organic coatings with self-healing ability to heal damaged (scratched) surfaces. In most cases, the intrinsic self-healing mechanism is preferred over the extrinsic self-healing mechanism because it can be used with transparent coatings and can heal scratches repeatedly[7,8]. However, high self-healing efficiency tends to adversely affect mechanical properties such as elastic modulus and hardness because the selfhealing process is strongly influenced by interdiffusion of polymer chains[9]. In particular,
automotive clearcoats require high durability; thus, achieving an appropriate balance between material properties and self-healing efficiency is an important factor in designing highperformance scratch-healing clearcoats. From this viewpoint, reversible polymer networks based on the dynamic hindered urea (HU) bond are a promising candidate for automotive scratch healing clearcoats due to the transparency, repeated self-healing properties and high mechanical strength of such coatings. Integration (high mechanical properties)–degradation (enhanced polymer chain mobility) properties of dynamic networks can be used to improve mechanical properties and self-healing efficiency[10]. The system can also be easily applied to an automotive coating process because commercial clearcoats are made from the polyacrylate graft copolymers containing hydroxyl groups and multifunctional isocyanates. In this paper, we designed a series of reversible polymer networks based on the dynamic HU bond for use in a self-healing clearcoat (Figure 1). Various diol-type HU adducts containing different oxyethylene branch units (HU-ethylene glycol, HU-EG; HU-tetra(ethylene glycol), HU-TEG; HU-poly(ethylene glycol), HU-PEG) were synthesized, blended with a conventional clear-coat binder (T30), and then crosslinked with hexamethylene diisocyanate trimer (Desmodur N3300). The materials properties of the clearcoats were controlled using different HU adducts and by adjusting their contents in the polymer networks. The material properties of the polymer coatings, such as the hardness (HIT), modulus (EIT), glass-transition temperature (Tg), and thermal stability (i.e., decomposition temperature, Td), were characterized using a nano-indentation tester (NI), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA), respectively. Finally, the plastic deformation, elastic recovery, and scratch-healing efficiency (enhanced by the dynamic HU bond) of the clearcoats were analyzed using a micro-scratch tester (MST) equipped with an optical microscope and subsequently correlated with the material properties of the corresponding polymer networks. 2. Experimental 2.1. Materials N,N ′ -Di-tert-butylethylenediamine (DtBuA), isophorone diisocyanate (IPDI), dibutyltin dilaurate (DBTDL), 1-hexanol, EG, TEG, PEG, and 2-butanone were purchased from Merck and were used as received. Acrylic polyol binder and polyisocyanate (Desmodur N3300) were kindly provided by NOROO BEE Chemical Co., Ltd.
2.2. Synthesis of diols containing a hindered urea group (HU-EG, HU-TEG, HU-PEG) To synthesized hindered urea adducts precursor (HU-IPDI), IPDI was dissolved in 2butanone, and DtBuA was added to the solution dropwise in 2:1 molar ratio with stirring at 35°C over a period of 2 h. Upon reaction completion, the HU-IPDI solution was slowly added to the corresponding diol solution in 2-butanone in 1:2 molar ratio in the presence of 1 wt% DBTDL with stirring at 70°C for 2 h; After solvent evaporation under reduced pressure, the final product was subsequently dried under vacuum. 2.3. Preparation of the clearcoats The acrylic polyol resin (T30), corresponding diol containing a HU group (HU-EG, HU-TEG, or HU-PEG), and Desmodur N3300 were mixed in equivalent OH : NCO molar ratios and dissolved in 2-butanone. The formulation is listed in Table 1. The solution was coated onto a basecoat-coated metal substrate using the drawdown bar coating method. The coated substrate was cured in an oven though a stepwise heating process (i.e., room temperature for 10 min, 50°C for 10 min, 70°C for 10 min, and 150°C for 20 min). 2.4. Chemical structure confirmation 1H-NMR
spectra of the synthesized materials were obtained using a 300 MHz NMR
spectrometer (Bruker, Ultrashield). CDCl3 (δ = 7.26 ppm) was employed as a deuterated solvent. Fourier transform infrared (FT-IR) spectra of the organic compounds were recorded over the wavenumber range 4000800 cm−1 using an FT-IR spectrometer (Thermo Fisher Scientific Inc., Nicolet 6700/Nicolet Continuum). The HU adducts in the self-healing polymer network form a reversible urea bond, which facilitates self-healing via an exchangeable covalent bonding mechanism. The dynamic reaction of the urea bond and subsequent side reaction of free isocyanate were confirmed by 1H-NMR analysis. To examine this phenomenon, we reacted HU-EG (1 mmol), 1-hexanol (2 mmol), and DBTDL (1 µL) in CDCl3 (5 mL) at 60°C. The samples for 1H-NMR analyses were collected immediately (0 h) and after 20 and 37 h of heating (Figure S1). In the 1H-NMR spectra, the characteristic peaks of (CH3)3NH-CH2group in DtBuA (b’) and -CH2OC(=O)NH- group in the urethane linkage (a’) newly appeared as the hindered amine-urethane exchange reaction (caused by the dynamic hindered urea bond) progressed.
2.5. Thermal and viscoelastic properties The thermal stabilities of the polymer networks were determined using TGA. The TGA (TA Instruments, TGA Q500) measurements were performed up to 700°C at a heating rate of 10°C min−1 under a N2 atmosphere. DSC (TA Instruments, DSC Q2000) measurements were carried out by heating from −40 to 80°C with heating rate of 10°C min−1 under a N2 environment. The Tg values of the crosslinked polymers were determined from the thermal transition measured during the second heating run. The DMA (TA Instruments, DMA Q800) measurements were performed to examine Tg values of the polymer films. A temperature scan was made in the range of −50 to 150 °C with a heating rate of 5 °C/min. The applied strain and frequency were 0.01% and 1 Hz, respectively. 2.6. Nano-indentation The load-displacement curves of the polymer networks were obtained using a nanoindentation tester (Anton Paar, NHT3) with a Berkovich-type indenter. The force was loaded to the polymer from 0 to 10 mN at a rate of 20 mN min−1, maintained at 10 mN for 10 s, and then unloaded in a process that reversed the loading process. The indentation hardness (HIT), and indentation modulus (EIT) values were calculated from the permanent depth of penetration (final depth, hf), maximum displacement (hmax), elastic unloading stiffness (S = dP/dh) values (Figures S2 and S3)[11]. 2.7. Scratch and scratch-healing tests Micro-scratch tester: Micro-scratch tests were conducted using a scratch test machine (Anton Paar, micro-scratch tester) with a Rockwell C indenter (tip radius 10 μm). In the prescan stage, the indenter was applied to the surface with a low load of 2 mN to record the surface profile. During scratching, a constant force (50 mN) was applied to the coating surface at a rate of 4 mm min−1. The lengths of scratches were 2 mm. After recording the scratch position and topography data, we evaluated the scratch-healing performances of the coatings by heating the scratched surfaces (75°C for 24 h) using a Peltier module installed in the scratch test machine. After the scratch-healing process, the healed locations were analyzed by optical microscopy (OM).
Razor-blade scratch tester: A micro-stage (Kwonsys, Korea) equipped with a razor blade (ST300, Dorco, Korea) was used to induce single scratches with a load of 5 mN. The scratched samples were then heated at 75°C for 24 h; the scratched and healed surfaces were subsequently examined by OM.
3. Results and discussion 3.1. Material design and synthesis In this study, we designed a series of polyacrylate urethane networks containing a HU group (C-HU-EG, C-HU-TEG, and C-HU-PEG) to create a scratch-healing clearcoat that exhibited dynamic covalent bonding. As a control, a commercial clearcoat (T30) was prepared, and its material properties and scratch-healing performance were compared with the corresponding properties of the synthesized self-healing polyacrylate urethane networks. The HU-EG, HUTEG, and HU-PEG were prepared through a two-step synthesis approach, as presented in Scheme 1. The chemical structure of the synthesized materials was confirmed using the 1HNMR and FT-IR spectroscopy (Figure 2(a)–(b)). In the 1H-NMR spectra, the characteristic peaks of –N(C(CH3)3)–CH2–, –CH2–NHC(=O)N(C(CH3)3)–, and –N(C(CH3)3)– groups in HU-IPDI appear at 3.3, 3.1, and 1.5 ppm, respectively. After urethane reaction of the HU-IPDI with diols, the characteristic peak of the –CH2–O–C(=O)NH– group additionally appears at 4.2 ppm[12]. The conversion of the reaction was also confirmed by monitoring the ratio between the intensity of the peak assigned to the methylene unit (-CH2-) at 2950–2850 cm−1 and that of the band assigned to the isocyanate unit (–NCO) at 2250 cm−1 in FT-IR spectra. 3.2. Thermal properties of the polymer networks The thermal stabilities of the synthesized clearcoats were investigated using TGA. All of the polymers were stable to at least 150°C (Figure 3). The Tg values of the polymers were determined using DSC and DMA (Figure 4 and S4). For C-HU-EG, with increasing HU-EG content in the polymer network, the Tg value increases because the polar urea bond and bulky cyclic chemical structure of IPDI restrict the polymer chain mobility[13,14]. However, C-HUPEG exhibits the opposite tendency as C-HU-EG. In this case, the effect of the flexible nature of PEG on the polymer chain mobility is greater than the effects of the other two factors (i.e., the hydrogen bonding of the urea bond and the bulky cyclic chemical structure of IPDI)[15].
In addition, with increasing HU-PEG content, the crosslinking density of the C-HU-PEG network decreases, thereby enhancing the polymer chain mobility[16]. Interestingly, in the case of C-HU-TEG, the Tg values are almost independent of the HU-TEG content in the polymer network until the HU-TEG content reaches 30 mol% but slightly increases when it reaches 50 mol%. This behavior is mainly attributed to the balanced chemical structure between the flexible unit (TEG) and the rigid unit (IPDI, urea, and urethane bond). 3.3. Mechanical properties of the polymer networks Figure 5 show load–displacement curves measured in the nano-indentation tests on the CHU-EG, C-HU-TEG, and C-HU-PEG polymer networks. The mechanical properties of the polymer networks were evaluated. All coatings exhibited plastic deformation when subjected to the applied normal forces; however, the HIT values of the polymer networks decreased with increasing content of oligo-oxyethylene units in the polymer networks (HIT(C-HU-EG) > HIT(C-HU-TEG) > HIT(C-HU-PEG)). The HIT values of the C-HU-EG polymer networks increased with increasing HU-EG content (HIT(C-HU-EG10) < HIT(C-HU-EG30) < HIT(C-HUEG50)), whereas the HIT values of the C-HU-PEG polymer networks decreased with increasing HU-PEG content (HIT(C-HU-PEG10) > HIT(C-HU-PEG30) > HIT(C-HU-PEG50)). All of these results are mainly attributed to differences in the crosslinking densities of the polymer networks[17]. The polymer networks of the HU-PEG, with long oligo-oxyethylene units, exhibit lower crosslinking densities, whereas those of the HU-EG, with short oxyethylene units, exhibit greater crosslinking densities. However, the HIT values of the C-HU-TEG polymer networks are almost independent of the HU-TEG content because of its balanced flexible unit– rigid unit chemical structure, as described in the previous section. The EIT values of the polymer networks showed similar trends as the HIT values. From the thermal and mechanical properties data, we confirmed that the C-HU-EG and C-HU-TEG polymer networks have material properties better than or similar to those of the commercial clearcoat (T30). 3.4. Scratch and scratch healing in the clearcoat systems Scratch tests of the C-HU-EG and C-HU-TEG polymers were performed using a microscratch tester. As clearly observed in the load vs. penetration plot for the C-HU-EG and C-HUTEG coatings (Figure 6), the penetration depth of the polymer coatings increased with decreasing EIT and HIT values of the polymers, which we attributed to their lower crosslinking
density. By contrast, the percentage recovery of the polymer coatings was inversely proportional to the polymers’ EIT and HIT values. The elastic factor (Ef) and plastic factor (Pf) values of the polymer coatings were calculated from the equations 𝑅𝑑
𝑃𝑓 =
𝑃𝑑
(1)
𝐸𝑓 = 1 ― 𝑃𝑓 (2) where Rd, Pd, Ef, and Pf represent the residual depth, penetration depth, elastic factor, and the plastic factor, respectively. The calculated values are listed in Table 2. The Ef values of the C-HU-EG polymer decreased with increasing HU-EG content in the polymer networks, whereas the changes in the Ef values of the C-HU-TEG polymers decreased with increasing HU-TEG content. This trend is consistent with the results of the nano-indentation tests and the thermal property measurements. The scratch recovery performance of the C-HU-EG and C-HU-TEG polymers was compared through analysis of OM images (Figure 7). The scratched polymer coatings were subjected to the target temperature (75°C) for 24 h using a Peltier heating module. The width profile data were used to calculate the scratch-width healing efficiency (%WSHE) values according to the equation
%𝑊𝑆𝐻𝐸 =
[
𝑊𝑖 ― 𝑊𝑟 𝑊𝑖
]𝑋 100 (3)
where Wi and Wr indicate the initial scratch width and the residual width, respectively[18]. The calculated %WSHE values are presented in Table 3. The scratched surfaces of the T30, C-HU-EG, and C-HU-TEG polymer coatings were healed at 75°C; however, the C-HU-TEG polymer coatings exhibited scratch-healing performances superior to those of the T30 and CHU-EG polymer coatings. In addition, the scratch-healing performances of the C-HU-TEG coatings increased in proportion to their HU-TEG content; however, their mechanical properties, such as their HIT and EIT, were independent of the crosslinking density of the polymer networks. By contrast, the self-healing performance of the C-HU-EG coatings decreased with increasing HU-EG content. These results indicate that the polymer chain mobility at the healing temperature, in addition to the dynamic covalent bond content and mechanical properties, is a critical factor governing scratch-healing performance[19].
Finally, the scratch and healing tests were performed in a practical environment using a razorblade scratch tester. The results show that C-HU-TEG30 exhibited superior scratch-healing performance compared with the commercial T30 clearcoat (Figure S5). 4. Conclusions This report describes the application of the dynamic HU-based scratch-healing polymer networks for use in automotive clearcoats. A series of polyacrylate urethane urea networks containing HU groups (C-HU-EG, C-HU-TEG, and C-HU-PEG) were prepared to investigate the effect of the materials properties, including the EIT, HIT, and the crosslinking density, on the polymer networks’ self-healing performance. The material properties and self-healing performance of the polymer networks were quantitatively characterized using TGA, DSC, DMA, NI, MST and OM. The analyses revealed that, among the investigated polymer networks, the C-HU-TEG30 polymer networks not only exhibited physical properties most similar to those of the commercial T30 polymer network but also demonstrated the best self-healing performance (Figure 8). Our results indicate that dynamic polymer networks based on a HUfunctionalized polymer are good candidates as scratch-healing systems for automotive clearcoats. In addition, the balanced chemical structure of the HU-TEG adduct between the flexible unit (TEG) and the rigid unit (IPDI, urea, and urethane bond) was useful to increase the self-healing performance of the polymer networks while maintaining the important materials properties for the automotive clearcoat such as the Tg, HIT and EIT values. Acknowledgements This study was supported by the Ministry of Trade, Industry & Energy of Korea (Industrial Technology Innovation Program No. 10067082 and Development of High Density Inks for High Speed Digital Transfer Printing No. 10078335). Reference [1] B. Ahmadi, M. Kassiriha, K. Khodabakhshi, E.R. Mafi, Effect of nano layered silicates on automotive polyurethane refinish clear coat, Progress in Organic Coatings. 60 (2007) 99–104. doi:10.1016/j.porgcoat.2007.07.008. [2] S.M. Noh, J.W. Lee, J.H. Nam, K.H. Byun, J.M. Park, H.W. Jung, Dual-curing behavior and scratch characteristics of hydroxyl functionalized urethane methacrylate oligomer for automotive clearcoats, Progress in Organic Coatings. 74 (2012) 257–269. doi:10.1016/j.porgcoat.2012.01.002.
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Fig. 1. (a) Illustration of material design and self-healing process of C-HU-EG, C-HU-TEG, and C-HU-PEG polymer networks, (b) Elastic modulus versus self-healing efficiency plots of the intrinsic self-healing materials.
1
Fig. 2. (a) H-NMR spectra of the synthesized HU-IPDI and HU-EG (in CDCl3) and (b) FTIR spectra of the HU-IPDI, HU-EG, HU-TEG, and HU-PEG.
Fig. 3. Thermal properties of synthesized polymer networks: TGA pyrogram.
Fig. 4. Thermal properties of the synthesized polymer networks: DSC thermograms for the (a) C-HU-EG, (b) C-HU-TEG, and (c) C-HU-PEG. Fig. 5. Load–displacement curves for (a) C-HU-EG, (b) C-HU-TEG, and (c) C-HU-PEG, as recorded in nano-indentation experiments; (d) calculated HIT and EIT values of polymer the coatings.
Fig. 6. Residual depth and penetration depth of the coating films (C-HU-EGs and C-HU-TEGs) scratched under a constant force 50 mN.
Fig. 7. Optical microscopy images obtained after the scratch tests, where specimens were scratched and then healed at 75°C for 24 h under a constant force (50 mN) applied to the polymer coatings: (a) T30, (b) C-HU-EG10, (c) C-HU-EG30, (d) C-HU-EG50, (e) C-HUTEG10 (f) C-HU-TEG30 and (g) C-HU-TEG50.
Fig. 8. 3-dimensional plot of glass transition temperature and elastic modulus versus healing efficiency of the polymer networks. Scheme. 1. Synthesis scheme and chemical structures of (a) HU-EG, HU-TEG, and HU-PEG, (b) acryl polyol, and (c) polyisocyanate (Desmodur N3300).
Table 1. Formulation of the polymer networks. Acryl
Polymer
Polyol
code
Hindered Urea Adduct
(g) HU-EG
HU-TEG
HU-PEG
(g)
(g)
(g)
OH
N3300
(mol%)
(g)
T30
10
-
-
-
-
3.32
C-HU-EG10
10
0.51
-
-
10
3.49
C-HU-EG30
10
1.96
-
-
30
3.97
C-HU-EG50
10
4.57
-
-
50
4.84
C-HU-TEG10
10
-
0.69
-
10
3.55
C-HU-TEG30
10
-
2.66
-
30
4.20
C-HU-TEG50
10
-
6.21
-
50
5.38
C-HU-PEG10
10
-
-
0.97
10
3.64
C-HU-PEG30
10
-
-
3.74
30
4.56
C-HU-PEG50
10
-
-
8.73
50
6.22
* The OH (mol%) indicates the molar percentage of hydroxyl groups in the hindered urea adduct to total hydroxyl groups in the corresponding polymer blend.
Table 2. Ef and Pf values of the polymer coatings at room temperature. Polymer code
Ef
Pf
T30
0.67
0.33
C-HU-EG10
0.74
0.26
C-HU-EG30
0.55
0.45
C-HU-EG50
0
1
C-HU-TEG10
0.80
0.20
C-HU-TEG30
0.74
0.26
C-HU-TEG50
0.74
0.26
Table 3. Dimensions of the scratched and healed areas and %WSHE values of the polymer networks heated at 75°C for 24 h. Scratch width (μm) Polymer code
%WSHE Initial
Healed
T30
35.6
11.3
68
C-HU-EG10
29.4
11.3
61
C-HU-EG30
28.7
11.3
60
C-HU-EG50
28.1
14.4
48
C-HU-TEG10
33.1
5.6
83
C-HU-TEG30
33.1
0
100
C-HU-TEG50
33.8
0
100
S0169-4332(19)33362-8 https://doi.org/10.1016/j.apsusc.2019.144546 APSUSC 144546
To appear in:
Applied Surface Science
Received Date: Accepted Date:
18 October 2019 29 October 2019
Please cite this article as: G. Young Kim, S. Sung, M.P. Kim, S. Cheon Kim, S-H. Lee, Y. Il Park, S. Man Noh, I. Woo Cheong, J. Chul Kim, Reversible Polymer Networks Based on the Dynamic Hindered Urea Bond for Scratch Healing in Automotive Clearcoats, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144546
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Reversible Polymer Networks Based on the Dynamic Hindered Urea Bond for Scratch Healing in Automotive Clearcoats Gi Young Kima,b, Sujin Sunga, Minsoo P. Kimc, Soon Cheon Kima, Sang-Ho Leea, Young Il Parka, Seung Man Noha, In Woo Cheong1,*, Jin Chul Kim1,* aResearch
Center for Green Fine Chemicals, Korea Research Institute of Chemical
Technology, Ulsan 44412, Republic of Korea bSchool
of Applied Chemical Engineering, Graduate School of Advanced Integration of
Science and Technology, Kyungpook National University, Daegu 41566, Republic of Korea cSchool
of Energy and Chemical Engineering, Ulsan National Institute of Technology
(UNIST), Ulsan 44919, Republic of Korea
*Corresponding
1I.
author.
W. Cheong and J. C. Kim are the co-corresponding authors of this article.
Highlights
• Synthesis of dynamic polyacrylate urethane urea networks containing hindered urea groups. • Control of the material properties of the polymer networks using different HU adducts. • Quantitative evaluation of the scratch-healing performances using a micro-scratch tester. • Excellent mechanical and scratch-resistant properties • Better scratch-healable performance over commercial clear coats
Abstract: A series of dynamic polyacrylate urethane urea networks containing hindered urea (HU) groups (C-HU-ethylene glycol, C-HU-EG; C-HU-tetra(ethylene glycol), C-HU-TEG; CHU-poly(ethylene glycol), C-HU-PEG) were successfully synthesized for use in automotive clearcoats. The material properties of the clearcoats were controlled using different HU adducts and by adjusting their contents in the polymer networks. The indentation hardness (HIT), modulus (EIT), thermal stability (Td), and glass-transition temperature (Tg) of the polymer
networks were characterized by nano-indentation tests, dynamic mechanical analysis, differential scanning calorimetry, and thermogravimetric analysis, respectively. The scratchresistance and healing performances of the polymer networks were evaluated quantitatively using a micro-scratch tester in conjunction with optical microscopy. The results reveal that the C-HU-TEG polymer networks not only exhibited physical properties most similar to those of the commercial clearcoat (T30) but also demonstrated the best self-healing performance. The balanced chemical structure of the HU-TEG adduct between the flexible unit (TEG) and the rigid unit (IPDI, urea, and urethane bond) played an important role of increasing scratchhealing performance while maintaining the clearcoat material properties.
Keywords: Intrinsic self-healing, Dynamic hindered urea bond, Dynamic polymer network, Automotive clearcoat, Scratch resistance, Scratch healing
1. Introduction Automotive clearcoats are transparent coatings that protect the substrate surface from harsh outdoor environments for prolonged periods. In general, automotive clearcoats are poly(urethane acrylate) networks with high crosslinking density so that they exhibit good longterm mechanical stability and chemical resistance[1,2]. However, because automotive clearcoats are based on organic polymeric materials, they are still vulnerable to scratching by sharp materials such as inorganic particles and metals. To overcome this drawback, considerable research effort has been devoted to developing coatings with high scratch resistance by increasing the polymer crosslinking density and/or adding inorganic materials. However, this approach is limited in applications that require high elasticity, processability, compatibility, and/or optical transparency[3–6]. Recently, several groups have attempted to impart organic coatings with self-healing ability to heal damaged (scratched) surfaces. In most cases, the intrinsic self-healing mechanism is preferred over the extrinsic self-healing mechanism because it can be used with transparent coatings and can heal scratches repeatedly[7,8]. However, high self-healing efficiency tends to adversely affect mechanical properties such as elastic modulus and hardness because the selfhealing process is strongly influenced by interdiffusion of polymer chains[9]. In particular,
automotive clearcoats require high durability; thus, achieving an appropriate balance between material properties and self-healing efficiency is an important factor in designing highperformance scratch-healing clearcoats. From this viewpoint, reversible polymer networks based on the dynamic hindered urea (HU) bond are a promising candidate for automotive scratch healing clearcoats due to the transparency, repeated self-healing properties and high mechanical strength of such coatings. Integration (high mechanical properties)–degradation (enhanced polymer chain mobility) properties of dynamic networks can be used to improve mechanical properties and self-healing efficiency[10]. The system can also be easily applied to an automotive coating process because commercial clearcoats are made from the polyacrylate graft copolymers containing hydroxyl groups and multifunctional isocyanates. In this paper, we designed a series of reversible polymer networks based on the dynamic HU bond for use in a self-healing clearcoat (Figure 1). Various diol-type HU adducts containing different oxyethylene branch units (HU-ethylene glycol, HU-EG; HU-tetra(ethylene glycol), HU-TEG; HU-poly(ethylene glycol), HU-PEG) were synthesized, blended with a conventional clear-coat binder (T30), and then crosslinked with hexamethylene diisocyanate trimer (Desmodur N3300). The materials properties of the clearcoats were controlled using different HU adducts and by adjusting their contents in the polymer networks. The material properties of the polymer coatings, such as the hardness (HIT), modulus (EIT), glass-transition temperature (Tg), and thermal stability (i.e., decomposition temperature, Td), were characterized using a nano-indentation tester (NI), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA), respectively. Finally, the plastic deformation, elastic recovery, and scratch-healing efficiency (enhanced by the dynamic HU bond) of the clearcoats were analyzed using a micro-scratch tester (MST) equipped with an optical microscope and subsequently correlated with the material properties of the corresponding polymer networks. 2. Experimental 2.1. Materials N,N ′ -Di-tert-butylethylenediamine (DtBuA), isophorone diisocyanate (IPDI), dibutyltin dilaurate (DBTDL), 1-hexanol, EG, TEG, PEG, and 2-butanone were purchased from Merck and were used as received. Acrylic polyol binder and polyisocyanate (Desmodur N3300) were kindly provided by NOROO BEE Chemical Co., Ltd.
2.2. Synthesis of diols containing a hindered urea group (HU-EG, HU-TEG, HU-PEG) To synthesized hindered urea adducts precursor (HU-IPDI), IPDI was dissolved in 2butanone, and DtBuA was added to the solution dropwise in 2:1 molar ratio with stirring at 35°C over a period of 2 h. Upon reaction completion, the HU-IPDI solution was slowly added to the corresponding diol solution in 2-butanone in 1:2 molar ratio in the presence of 1 wt% DBTDL with stirring at 70°C for 2 h; After solvent evaporation under reduced pressure, the final product was subsequently dried under vacuum. 2.3. Preparation of the clearcoats The acrylic polyol resin (T30), corresponding diol containing a HU group (HU-EG, HU-TEG, or HU-PEG), and Desmodur N3300 were mixed in equivalent OH : NCO molar ratios and dissolved in 2-butanone. The formulation is listed in Table 1. The solution was coated onto a basecoat-coated metal substrate using the drawdown bar coating method. The coated substrate was cured in an oven though a stepwise heating process (i.e., room temperature for 10 min, 50°C for 10 min, 70°C for 10 min, and 150°C for 20 min). 2.4. Chemical structure confirmation 1H-NMR
spectra of the synthesized materials were obtained using a 300 MHz NMR
spectrometer (Bruker, Ultrashield). CDCl3 (δ = 7.26 ppm) was employed as a deuterated solvent. Fourier transform infrared (FT-IR) spectra of the organic compounds were recorded over the wavenumber range 4000800 cm−1 using an FT-IR spectrometer (Thermo Fisher Scientific Inc., Nicolet 6700/Nicolet Continuum). The HU adducts in the self-healing polymer network form a reversible urea bond, which facilitates self-healing via an exchangeable covalent bonding mechanism. The dynamic reaction of the urea bond and subsequent side reaction of free isocyanate were confirmed by 1H-NMR analysis. To examine this phenomenon, we reacted HU-EG (1 mmol), 1-hexanol (2 mmol), and DBTDL (1 µL) in CDCl3 (5 mL) at 60°C. The samples for 1H-NMR analyses were collected immediately (0 h) and after 20 and 37 h of heating (Figure S1). In the 1H-NMR spectra, the characteristic peaks of (CH3)3NH-CH2group in DtBuA (b’) and -CH2OC(=O)NH- group in the urethane linkage (a’) newly appeared as the hindered amine-urethane exchange reaction (caused by the dynamic hindered urea bond) progressed.
2.5. Thermal and viscoelastic properties The thermal stabilities of the polymer networks were determined using TGA. The TGA (TA Instruments, TGA Q500) measurements were performed up to 700°C at a heating rate of 10°C min−1 under a N2 atmosphere. DSC (TA Instruments, DSC Q2000) measurements were carried out by heating from −40 to 80°C with heating rate of 10°C min−1 under a N2 environment. The Tg values of the crosslinked polymers were determined from the thermal transition measured during the second heating run. The DMA (TA Instruments, DMA Q800) measurements were performed to examine Tg values of the polymer films. A temperature scan was made in the range of −50 to 150 °C with a heating rate of 5 °C/min. The applied strain and frequency were 0.01% and 1 Hz, respectively. 2.6. Nano-indentation The load-displacement curves of the polymer networks were obtained using a nanoindentation tester (Anton Paar, NHT3) with a Berkovich-type indenter. The force was loaded to the polymer from 0 to 10 mN at a rate of 20 mN min−1, maintained at 10 mN for 10 s, and then unloaded in a process that reversed the loading process. The indentation hardness (HIT), and indentation modulus (EIT) values were calculated from the permanent depth of penetration (final depth, hf), maximum displacement (hmax), elastic unloading stiffness (S = dP/dh) values (Figures S2 and S3)[11]. 2.7. Scratch and scratch-healing tests Micro-scratch tester: Micro-scratch tests were conducted using a scratch test machine (Anton Paar, micro-scratch tester) with a Rockwell C indenter (tip radius 10 μm). In the prescan stage, the indenter was applied to the surface with a low load of 2 mN to record the surface profile. During scratching, a constant force (50 mN) was applied to the coating surface at a rate of 4 mm min−1. The lengths of scratches were 2 mm. After recording the scratch position and topography data, we evaluated the scratch-healing performances of the coatings by heating the scratched surfaces (75°C for 24 h) using a Peltier module installed in the scratch test machine. After the scratch-healing process, the healed locations were analyzed by optical microscopy (OM).
Razor-blade scratch tester: A micro-stage (Kwonsys, Korea) equipped with a razor blade (ST300, Dorco, Korea) was used to induce single scratches with a load of 5 mN. The scratched samples were then heated at 75°C for 24 h; the scratched and healed surfaces were subsequently examined by OM.
3. Results and discussion 3.1. Material design and synthesis In this study, we designed a series of polyacrylate urethane networks containing a HU group (C-HU-EG, C-HU-TEG, and C-HU-PEG) to create a scratch-healing clearcoat that exhibited dynamic covalent bonding. As a control, a commercial clearcoat (T30) was prepared, and its material properties and scratch-healing performance were compared with the corresponding properties of the synthesized self-healing polyacrylate urethane networks. The HU-EG, HUTEG, and HU-PEG were prepared through a two-step synthesis approach, as presented in Scheme 1. The chemical structure of the synthesized materials was confirmed using the 1HNMR and FT-IR spectroscopy (Figure 2(a)–(b)). In the 1H-NMR spectra, the characteristic peaks of –N(C(CH3)3)–CH2–, –CH2–NHC(=O)N(C(CH3)3)–, and –N(C(CH3)3)– groups in HU-IPDI appear at 3.3, 3.1, and 1.5 ppm, respectively. After urethane reaction of the HU-IPDI with diols, the characteristic peak of the –CH2–O–C(=O)NH– group additionally appears at 4.2 ppm[12]. The conversion of the reaction was also confirmed by monitoring the ratio between the intensity of the peak assigned to the methylene unit (-CH2-) at 2950–2850 cm−1 and that of the band assigned to the isocyanate unit (–NCO) at 2250 cm−1 in FT-IR spectra. 3.2. Thermal properties of the polymer networks The thermal stabilities of the synthesized clearcoats were investigated using TGA. All of the polymers were stable to at least 150°C (Figure 3). The Tg values of the polymers were determined using DSC and DMA (Figure 4 and S4). For C-HU-EG, with increasing HU-EG content in the polymer network, the Tg value increases because the polar urea bond and bulky cyclic chemical structure of IPDI restrict the polymer chain mobility[13,14]. However, C-HUPEG exhibits the opposite tendency as C-HU-EG. In this case, the effect of the flexible nature of PEG on the polymer chain mobility is greater than the effects of the other two factors (i.e., the hydrogen bonding of the urea bond and the bulky cyclic chemical structure of IPDI)[15].
In addition, with increasing HU-PEG content, the crosslinking density of the C-HU-PEG network decreases, thereby enhancing the polymer chain mobility[16]. Interestingly, in the case of C-HU-TEG, the Tg values are almost independent of the HU-TEG content in the polymer network until the HU-TEG content reaches 30 mol% but slightly increases when it reaches 50 mol%. This behavior is mainly attributed to the balanced chemical structure between the flexible unit (TEG) and the rigid unit (IPDI, urea, and urethane bond). 3.3. Mechanical properties of the polymer networks Figure 5 show load–displacement curves measured in the nano-indentation tests on the CHU-EG, C-HU-TEG, and C-HU-PEG polymer networks. The mechanical properties of the polymer networks were evaluated. All coatings exhibited plastic deformation when subjected to the applied normal forces; however, the HIT values of the polymer networks decreased with increasing content of oligo-oxyethylene units in the polymer networks (HIT(C-HU-EG) > HIT(C-HU-TEG) > HIT(C-HU-PEG)). The HIT values of the C-HU-EG polymer networks increased with increasing HU-EG content (HIT(C-HU-EG10) < HIT(C-HU-EG30) < HIT(C-HUEG50)), whereas the HIT values of the C-HU-PEG polymer networks decreased with increasing HU-PEG content (HIT(C-HU-PEG10) > HIT(C-HU-PEG30) > HIT(C-HU-PEG50)). All of these results are mainly attributed to differences in the crosslinking densities of the polymer networks[17]. The polymer networks of the HU-PEG, with long oligo-oxyethylene units, exhibit lower crosslinking densities, whereas those of the HU-EG, with short oxyethylene units, exhibit greater crosslinking densities. However, the HIT values of the C-HU-TEG polymer networks are almost independent of the HU-TEG content because of its balanced flexible unit– rigid unit chemical structure, as described in the previous section. The EIT values of the polymer networks showed similar trends as the HIT values. From the thermal and mechanical properties data, we confirmed that the C-HU-EG and C-HU-TEG polymer networks have material properties better than or similar to those of the commercial clearcoat (T30). 3.4. Scratch and scratch healing in the clearcoat systems Scratch tests of the C-HU-EG and C-HU-TEG polymers were performed using a microscratch tester. As clearly observed in the load vs. penetration plot for the C-HU-EG and C-HUTEG coatings (Figure 6), the penetration depth of the polymer coatings increased with decreasing EIT and HIT values of the polymers, which we attributed to their lower crosslinking
density. By contrast, the percentage recovery of the polymer coatings was inversely proportional to the polymers’ EIT and HIT values. The elastic factor (Ef) and plastic factor (Pf) values of the polymer coatings were calculated from the equations 𝑅𝑑
𝑃𝑓 =
𝑃𝑑
(1)
𝐸𝑓 = 1 ― 𝑃𝑓 (2) where Rd, Pd, Ef, and Pf represent the residual depth, penetration depth, elastic factor, and the plastic factor, respectively. The calculated values are listed in Table 2. The Ef values of the C-HU-EG polymer decreased with increasing HU-EG content in the polymer networks, whereas the changes in the Ef values of the C-HU-TEG polymers decreased with increasing HU-TEG content. This trend is consistent with the results of the nano-indentation tests and the thermal property measurements. The scratch recovery performance of the C-HU-EG and C-HU-TEG polymers was compared through analysis of OM images (Figure 7). The scratched polymer coatings were subjected to the target temperature (75°C) for 24 h using a Peltier heating module. The width profile data were used to calculate the scratch-width healing efficiency (%WSHE) values according to the equation
%𝑊𝑆𝐻𝐸 =
[
𝑊𝑖 ― 𝑊𝑟 𝑊𝑖
]𝑋 100 (3)
where Wi and Wr indicate the initial scratch width and the residual width, respectively[18]. The calculated %WSHE values are presented in Table 3. The scratched surfaces of the T30, C-HU-EG, and C-HU-TEG polymer coatings were healed at 75°C; however, the C-HU-TEG polymer coatings exhibited scratch-healing performances superior to those of the T30 and CHU-EG polymer coatings. In addition, the scratch-healing performances of the C-HU-TEG coatings increased in proportion to their HU-TEG content; however, their mechanical properties, such as their HIT and EIT, were independent of the crosslinking density of the polymer networks. By contrast, the self-healing performance of the C-HU-EG coatings decreased with increasing HU-EG content. These results indicate that the polymer chain mobility at the healing temperature, in addition to the dynamic covalent bond content and mechanical properties, is a critical factor governing scratch-healing performance[19].
Finally, the scratch and healing tests were performed in a practical environment using a razorblade scratch tester. The results show that C-HU-TEG30 exhibited superior scratch-healing performance compared with the commercial T30 clearcoat (Figure S5). 4. Conclusions This report describes the application of the dynamic HU-based scratch-healing polymer networks for use in automotive clearcoats. A series of polyacrylate urethane urea networks containing HU groups (C-HU-EG, C-HU-TEG, and C-HU-PEG) were prepared to investigate the effect of the materials properties, including the EIT, HIT, and the crosslinking density, on the polymer networks’ self-healing performance. The material properties and self-healing performance of the polymer networks were quantitatively characterized using TGA, DSC, DMA, NI, MST and OM. The analyses revealed that, among the investigated polymer networks, the C-HU-TEG30 polymer networks not only exhibited physical properties most similar to those of the commercial T30 polymer network but also demonstrated the best self-healing performance (Figure 8). Our results indicate that dynamic polymer networks based on a HUfunctionalized polymer are good candidates as scratch-healing systems for automotive clearcoats. In addition, the balanced chemical structure of the HU-TEG adduct between the flexible unit (TEG) and the rigid unit (IPDI, urea, and urethane bond) was useful to increase the self-healing performance of the polymer networks while maintaining the important materials properties for the automotive clearcoat such as the Tg, HIT and EIT values. Acknowledgements This study was supported by the Ministry of Trade, Industry & Energy of Korea (Industrial Technology Innovation Program No. 10067082 and Development of High Density Inks for High Speed Digital Transfer Printing No. 10078335). Reference [1] B. Ahmadi, M. Kassiriha, K. Khodabakhshi, E.R. Mafi, Effect of nano layered silicates on automotive polyurethane refinish clear coat, Progress in Organic Coatings. 60 (2007) 99–104. doi:10.1016/j.porgcoat.2007.07.008. [2] S.M. Noh, J.W. Lee, J.H. Nam, K.H. Byun, J.M. Park, H.W. Jung, Dual-curing behavior and scratch characteristics of hydroxyl functionalized urethane methacrylate oligomer for automotive clearcoats, Progress in Organic Coatings. 74 (2012) 257–269. doi:10.1016/j.porgcoat.2012.01.002.
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Fig. 1. (a) Illustration of material design and self-healing process of C-HU-EG, C-HU-TEG, and C-HU-PEG polymer networks, (b) Elastic modulus versus self-healing efficiency plots of the intrinsic self-healing materials.
1
Fig. 2. (a) H-NMR spectra of the synthesized HU-IPDI and HU-EG (in CDCl3) and (b) FTIR spectra of the HU-IPDI, HU-EG, HU-TEG, and HU-PEG.
Fig. 3. Thermal properties of synthesized polymer networks: TGA pyrogram.
Fig. 4. Thermal properties of the synthesized polymer networks: DSC thermograms for the (a) C-HU-EG, (b) C-HU-TEG, and (c) C-HU-PEG. Fig. 5. Load–displacement curves for (a) C-HU-EG, (b) C-HU-TEG, and (c) C-HU-PEG, as recorded in nano-indentation experiments; (d) calculated HIT and EIT values of polymer the coatings.
Fig. 6. Residual depth and penetration depth of the coating films (C-HU-EGs and C-HU-TEGs) scratched under a constant force 50 mN.
Fig. 7. Optical microscopy images obtained after the scratch tests, where specimens were scratched and then healed at 75°C for 24 h under a constant force (50 mN) applied to the polymer coatings: (a) T30, (b) C-HU-EG10, (c) C-HU-EG30, (d) C-HU-EG50, (e) C-HUTEG10 (f) C-HU-TEG30 and (g) C-HU-TEG50.
Fig. 8. 3-dimensional plot of glass transition temperature and elastic modulus versus healing efficiency of the polymer networks. Scheme. 1. Synthesis scheme and chemical structures of (a) HU-EG, HU-TEG, and HU-PEG, (b) acryl polyol, and (c) polyisocyanate (Desmodur N3300).
Table 1. Formulation of the polymer networks. Acryl
Polymer
Polyol
code
Hindered Urea Adduct
(g) HU-EG
HU-TEG
HU-PEG
(g)
(g)
(g)
OH
N3300
(mol%)
(g)
T30
10
-
-
-
-
3.32
C-HU-EG10
10
0.51
-
-
10
3.49
C-HU-EG30
10
1.96
-
-
30
3.97
C-HU-EG50
10
4.57
-
-
50
4.84
C-HU-TEG10
10
-
0.69
-
10
3.55
C-HU-TEG30
10
-
2.66
-
30
4.20
C-HU-TEG50
10
-
6.21
-
50
5.38
C-HU-PEG10
10
-
-
0.97
10
3.64
C-HU-PEG30
10
-
-
3.74
30
4.56
C-HU-PEG50
10
-
-
8.73
50
6.22
* The OH (mol%) indicates the molar percentage of hydroxyl groups in the hindered urea adduct to total hydroxyl groups in the corresponding polymer blend.
Table 2. Ef and Pf values of the polymer coatings at room temperature. Polymer code
Ef
Pf
T30
0.67
0.33
C-HU-EG10
0.74
0.26
C-HU-EG30
0.55
0.45
C-HU-EG50
0
1
C-HU-TEG10
0.80
0.20
C-HU-TEG30
0.74
0.26
C-HU-TEG50
0.74
0.26
Table 3. Dimensions of the scratched and healed areas and %WSHE values of the polymer networks heated at 75°C for 24 h. Scratch width (μm) Polymer code
%WSHE Initial
Healed
T30
35.6
11.3
68
C-HU-EG10
29.4
11.3
61
C-HU-EG30
28.7
11.3
60
C-HU-EG50
28.1
14.4
48
C-HU-TEG10
33.1
5.6
83
C-HU-TEG30
33.1
0
100
C-HU-TEG50
33.8
0
100