Stiff Self-Healing Coating Based on UV-Curable Polyurethane with a "Hard Core, Flexible Arm" Structure.

Two series of ultraviolet (UV)-cured self-healing polyurethane (PU) oligomers were synthesized through a prepolymer process from isophorone diisocyanate (IPDI) or 1,6-hexamethylene diisocyanate (HDI), polycarbonate diol (PCDL) of varying molecular weight (500, 1000, and 2000 Da), and chemically modified cyclotriphosphazene as hard cores were introduced. The synthesized oligomers contained rigid aromatic rings as "hard cores" and long fatty chains as "flexible arms". Nuclear magnetic resonance spectrometer (H NMR) and Fourier transform infrared spectroscopy were used to characterize the structures of the oligomers. In addition, the UV-cured self-healing PU coatings were prepared by designing some coating formulations with the PU oligomers. The self-healing properties and mechanical properties of the UV-cured coatings were investigated. The results revealed that the coatings had self-healing properties based on hydrogen bonds. As the molecular weight of PCDL decreased, the coatings exhibited increased hardness, tensile strength, and glass transition temperature. Furthermore, the coatings exhibited excellent thermostability. The results proved the application prospects of the self-healing coatings with high repair efficiency and excellent mechanical properties.

Introduction

The ability to repair itself after being damaged, which is defined “self-healing”, is an common phenomenon in nature.[1,2] The development of self-healing coatings is highly desirable because it greatly enhances the durability and longevity of the coating while reducing the adverse effects of the surrounding environment, such as physical and chemical damage.[3−8] Thus far, two kinds of self-healing coatings have been developed: (I) extrinsic self-healing coatings, need to pre-embed the healing agent firstly and (II) intrinsic self-healing coatings that can repair damage by reversible physical or chemical interactions of the polymer itself.[9−13] For the extrinsic self-healing, the damage can only be repaired once at the specific location, whereas the intrinsic self-healing could heal itself multiple times. Therefore, in recent years, more and more researchers have paid attention to intrinsic self-healing materials and have achieved many achievements.[14−17]

Many types of polymers have been used in the study of self-healing coatings, and the mobility of polymer chains is an important factor in ensuring self-healing ability.[18] The healing can only be achieved when the molecular chain diffuses to the damage interface.[19,20] Therefore, most existing self-healing polymers are soft owing to their low glass transition temperature (Tg). However, a self-healing coating characterized by high hardness is required for industrial and aerospace applications. To solve this problem, Guan et al.[21] reported a novel self-healing polymer that could achieve repair of damage without any external stimulation. Structurally, this polymer contains the hard–soft microphase-separated system. The acetylamino polymer as soft phase provides the reversible hydrogen bonding for self-healing, and the rigid polystyrene as hard phase provides strength to the material. This is an effective approach to solving the problem associated with intrinsic self-healing coatings.

Recently, a few publications have reported the polymers with “hard core, flexible arm” structures. Our research group successfully prepared ultraviolet (UV)-curable cardanol-based polymers with “hard core, flexible shell” structures to be used as coatings. The surface mechanical properties of these coatings, such as hardness, reduced modulus, surface storage modulus, and storage stiffness, constituted a considerable improvement.[22] Liu and Chung reported a new functional polymer containing lignin, lignin-graft-poly(5-acetylaminopentyl acrylate) (lignin-graft-PAA), a terminally functionalized polymer obtained by covalently bonding chemically modified lignin to PAA. The polymer had both a rigid lignin phase and a rubber-like PAA soft phase, but the two could be well dispersed to form a multiphase microstructure. The samples also exhibited good self-healing properties and recovered to 93% of the maximum tensile stress of the original sample after repair.[23] Therefore, the systematic study of the application of such polymers in self-healing coatings is of great significance.

In this paper, “hard core, flexible arm” polyurethane (PU) with photo-sensitive groups was synthesized by using the rigid group as the “core” and the flexible segment as the “arm”. The polymers were used to prepare a UV-cured self-healing coating and then investigated properties of the polymer coating, such as hardness, gloss, and adhesion. The results revealed that the polymer coatings exhibited good adhesion, gloss, and high repair efficiency. These results provide a new strategy for preparing self-healing coatings with good repair performance and excellent mechanical properties, thereby promoting the further development of related research.

Results and Discussion

Structural Characterization of UV-Cured Self-Healing PU

The 1H NMR spectra of IPH and HPH are shown in Figure , as depicted in the 1H NMR spectrum. The resonances of N–H were observed at 6.87–7.13 ppm, and the chemical shift of double bonds (C=C) was δ = 5.69 and 6.03 ppm, indicating that the photosensitive double bonds had been successfully grafted to the prepolymer based on the reaction between isocyanates and hydroxyl[27] and the signals within the 7.14–7.27 and 6.76–6.85 ppm ranges that were connected to the benzene ring in the cyclotriphosphazene monomer. The product was successfully synthesized. Water was an impurity doped in the process of purification. Simultaneously combined with Fourier transform infrared spectroscopy (FTIR) spectrum to monitor the reaction process and the structure of IPH or HPH, the FTIR spectra are shown in Figure S3.

Figure 1

1H NMR spectra of (a) IP500H6 and (b) HP500H6.

Table shows the molecular weights and polydispersity indices (PDIs) of synthesized PU samples. The addition of polycarbonate diol (PCDL) with a high molecular weight will further generate longer flexible segments, resulting in an increase in the molecular weight of PU.

Table 1

Compositions and Molecular Weight Data of the IPH6 and HPH6

		soft chain
sample	hard core	PCDL	isocyanate	photosensitive group	Mn	Mw	PDI
IP500H6	H6	500	IPDI	HEMA	5800	10 350	1.78
IP1000H6	H6	1000	IPDI	HEMA	7010	10 080	1.43
IP2000H6	H6	2000	IPDI	HEMA	12 050	20 220	1.68
HP500H6	H6	500	HDI	HEMA	3980	5640	1.41
HP1000H6	H6	1000	HDI	HEMA	5630	8290	2.04
HP2000H6	H6	2000	HDI	HEMA	8660	15 830	1.76

Characterization of Hydrogen Bonds

We envisioned that in our “hard core, flexible arm” structural design, the self-healing properties of polymer were due to the presence of reversible hydrogen bonds in the soft arm. Therefore, we performed a further in-depth investigation of the hydrogen bond through nuclear magnetic shift and rheological spectra of samples at different temperatures refer to previous studies.[14,28]Figure showed the 1H NMR spectra of HP500H6 at four temperatures (25, 50, 75, and 100 °C), as can be seen from the spectra, the N–H shifts will change with the temperature. When the temperature increased, the chemical shift of N–H moved to a lower field, indicating the existence of reversible hydrogen bonds in our polymer network. At higher temperatures, more hydrogen bonds were destroyed, the density of electron clouds around protons increased, as a result, enhanced the shielding effect and reduced the chemical shift. Based on these observations, when the chain segment moved to the damaged place, the hydrogen bond was formed again to repair the scratch. The 1H NMR spectra of IP500H6 at four temperatures (25, 50, 75, and 100 °C) shown in Figure S4 all exhibit the same trend.

Figure 2

1H NMR spectra of HP500H6 of different temperature.

There were two relaxation modes for characterizing the melt-rheology properties of hydrogen-bonded polymers, one was due to the polymer itself and another was due to the reversibility hydrogen bonding in the polymer, which could be opened and closed with thermo-reversibly. As shown in Figures and S5,[29,30] we investigated the rheological properties of HP500H6 and IP500H6 by measuring the change of loss modulus with frequency at different temperatures. The loss modulus was found to have a significant frequency dependence, when the frequency increased, the loss modulus gradually increased, and when the temperature rose, it gradually decreased. This could be attributable to temperature rise caused hydrogen bond rupture, which affected the dependence of loss modulus on frequency. In addition, the temperature increase perturbed the equilibrium and kinetics of the hydrogen bond association, thereby minimizing the effects on the loss modulus. This result combined with the chemical shift of hydrogen bonds in 1H NMR at various temperatures could prove the existence of hydrogen bond networks in polymers.

Figure 3

Loss modulus master curves of HP500H6 at various temperatures and frequencies.

Self-Healing Properties of Coatings

When the surface of coating was damaged, the hydrogen bond between the chains was broken by heating. At the same time, with an increase in temperature, the mobility of the soft segment increased and enables the chains to move to the broken interface. Finally, the coating cooled down, and through the reconstruction of hydrogen networks, the surface healed according to the design shown in Figure . The self-healing performance of coatings based on IPH or HPH were investigated through ultra-deep-field three-dimensional (3D) microscopy and atomic force microscopy (AFM). A razor was used to scratch a series of sample, followed by heating to 100 °C to repair it subsequently. From a series of data, it can be seen that the scratch of the self-healing coating could be almost fully repaired.

Figure 4

Schematic diagram of the repair process of self-healing coating.

The self-healing property was evaluated through a scratch repair test first. The scratched coatings on a heating plate were placed at the specified temperature for a specified period, and the scratch repair process was monitored through optical microscopy; from Figure we could see the process. For the coatings based on IPH, that based on IP500H6 or IP1000H6 had the optimal healing ability; surface damage to this coating disappeared after the coating was heated at 100 °C. The presence of hydrogen bonding in PU provided shape memory and healing ability, and the soft chain provided flexibility when the coating was healing.[30] However, the healing performance of IP2000H6 was unclear because of a lower hydrogen bond proportion in the IP2000H6 coating. At the same time, the softer segments caused the entanglement of chains and reduced chain mobility. In addition, the coating based on HP500H6 or HP1000H6 had excellent healing ability. By contrast, the HP2000H6 had a longer polymer chain, which caused entanglement between the segments, making it difficult to diffuse to the scratch interface, resulting in poor repair effect.

Figure 5

Optical microscope images of UV-cured self-healing coatings based on IPH or HPH before and after healing at 100 °C for periods of varying length. Before healing: (A) IP500H6; (B) IP1000H6; (C) IP2000H6; (D) IP500H6; (E) HP500H6; (F) HP1000H6; and (G) HP2000H6; and (H) HP500H6. After healing for (a) 5; (b) 5; (c) 60; (d) 5; (e) 5; (f) 5; (g) 60; and (h) 5 min.

We further explored the relatively high repair performance of the coatings based on IP500H6, IP1000H6, HP500H6, and HP1000H6. AFM images of these coatings captured before and after healing are shown in Figures and S6. All of these coatings were healed through the same way within 1 min. This was consistent with the above micrograph results, it proved that when the coating was damaged to some extent, the coatings could achieve the self-healing effect. The healing process could be observed in Movies S1–S4.

Figure 6

AFM images of UV-cured self-healing coatings based on IPH and HPH after they had been damaged: (A) IP500H6; (B) IP1000H6; (C) HP500H6; and (D) HP1000H6; and the same coatings after healing at 100 °C for 1 min: (a); (b); (c); and (d), respectively.

Properties of the Cured Coatings

Regarding the hardness of the coating with a better repair effect, we further studied the hardness of a coating with excellent repair performance. When the molecular weight of PCDL increased (Table ), the basic properties of the coatings based on IPH6 and HPH6 decreased somewhat (Table ). In particular, the coatings based on IP500H6 had the highest pencil hardness of up to 2H and the highest pendulum hardness of up to 146 ± 2 s; these results could be attributed to the higher double-bond content supporting higher cross-linking densities in the IP500H6 coating. The higher proportion of the rigid group provided the coating with higher rigidity. In addition, the coatings had superior gloss and adhesion. As the molecular weight of PCDL increased, the soft segment of the oligomer became longer and the rigidity of the chain decreased. At the same time, the increase in the molecular weight caused a relative decrease in the content of double bond and the cross-linking point density was lower and the curried coating was softer. The coatings based on IPH6 had the higher hardness compared with HPH6 at the same molecular weight of PCDL. The HPH6 coating had low hardness, and the higher the molecular weight of PCDL, the lower the hardness is.

Table 2

Properties of Coatings Based on IPH or HPH

sample	pendulum hardness (s)	pencil hardness	gloss (60°)	adhesion
IP500H6	146 ± 2	2H	142 ± 1.6	0
IP1000H6	40 ± 1	HB	120 ± 0.6	3
IP2000H6	33 ± 1	B	93.5 ± 0.4	2
HP500H6	40 ± 1	HB	132 ± 2.7	1
HP1000H6	24 ± 2	B	96 ± 3.7	2
HP2000H6	17 ± 2	2B	90 ± 2.5	3

We further conducted preliminary research for the properties of the coatings based on IPH6 on different substrates (Table S1) that exhibited approximately the same trend.

Dynamic Thermomechanical Analysis of Self-Healing Coatings

Figure shows the variation in the tan δ as a function of the temperature for the IPH and HPH. The Tg values of the IPH and HPH are shown in Figure a,b. As the Mn of PCDL increased, the Tg value of the PU decreased. Regarding the coatings based on IPH, Tg decreased from 64.4 to −2 °C, and the Tg of HPH decreased from 15.5 to −28.2 °C. Increasing the molecular weight caused the Tg move to a lower temperature, which could be attributed to the decrease in the content of hard domain in the polymer network.[31,32] With the increase of the rigid structure of the polymer, the flexibility of PU chains decreased, and thus Tg increased with an increase in cyclotriphosphazene content. Because of the lower molecular weight of PCDL500, it can be well dissolved, be reacted with isocyanate to a higher degree, form an ideal polymer network, and achieve a series of performances we expected.

Figure 7

tan δ dependence on temperature for the coatings based on the PU: (a) IPH; (b) HPH.

Thermal Stability Characterization of the Coatings

The thermal stabilities of the coatings based on IPH and HPH was studied by thermal gravimetric analyzer measurements. From Figure , the thermal stability of the series of self-healing coatings could be found. The temperature of the coatings based on IP500H6, IP1000H6, IP2000H6, HP500H6 HP1000H6, and HP2000H6 5 wt % weight loss (T5%) were 261, 267, 273, 268, 269, and 259 °C, respectively, indicating that all samples had good thermal resistance at the temperatures below 259 °C (Table S2). From the above figure, we could not only see the thermal stability of the PU but also see the decomposition process of the hard segments and soft segments. PU chains consist of carbamate as a hard segment and polyols as a soft segment.[33] As we all know, compared with the long carbon chain structure in the soft segment, the urethane structure in the hard segment of the PU was more prone to be decomposed. Therefore, the decomposition of urethane hard segments could be considered as the first degradation section in afore mentioned oligomer; the second stage was the decomposition of long fat chains in the soft segment.[34] A minor increase in T5% showed that the increase in the Mn of the PCDLs will increase the heat resistance of the coatings. In addition, when the Mn of the PCDLs was 500, the amount of char residue at 600 °C was highest and the amount of residue decreased as the molecular weight increased. This phenomenon was due to the cyclotriphosphazene that contains a great deal of cyclic structures such as benzene rings to give it better heat resistance.

Figure 8

Thermogravimetric analysis (TGA) curves of the cured coatings based on the PU: (a) IPH; (b) HPH.

Mechanical Properties of Self-Healing Coatings

The mechanical properties of the coatings based on IPH and HPH were investigated through a tension test. From the Figure , we could see the stress–strain curves, tensile strength, and elongation at break. The coatings based on IPH exhibited superior mechanical properties to those based on HPH, especially at a molecular weight (PCDL) of 500.

Figure 9

Stress and strain curves of the PU coatings: (a) IPH; (b) HPH. Tensile strength (c) and elongation at break (d) of the two series of coatings based on the PU (the X-axis represents the Mn of PCDLs in PU).

Regarding tensile strength, Figure c shows different results. The tensile strength achieved a maximum value of 24.33 ± 3.91 MPa at IP500H6, which then decreased at a lower molecular weight of PCDL. The increasing rigidity of the chain was expected as the introduction of more benzene ring structure in the PU, and more double bonds were observed in the IP500H6 coatings to form a denser cross-linked structures, provided stronger intermolecular interaction. For the HPH series, the maximum tensile strength of 8.33 ± 0.87 MPa at HP500H6 was obtained. The tensile strength decreased with an increase in Mn in both of the two series of coatings, and the elongation at break increased. Figure d shows that more than 339% of the ultimate strain could be obtained for IP2000H6 because a high amount of elastomeric PCDL2000 was introduced into the systems. The introduction of a rigid structure in the polymer would effectively increase the hardness and strength of the coating; it would reduce the toughness of the coating due to suppressed plastic deformation upon deformation and reduced elongation at break.[35,36] High soft segment content in the polymer network improved the flexibility of the polymer.

Conclusion

In summary, we developed two series of UV-cured self-healing coatings based on PU with “hard core, flexible arm” structures. On the basis of a comparison between the coatings based on IPH and HPH, both coatings exhibited excellent repair effect, the soft arm provided mobility in the chains, and reversible hydrogen bonding was used to achieve repair performance. Although the coating based on IP500H6 had higher hardness, this can be attributed to the rigid structure of the hard core and polymer networks formed by double bonds, and thus the hardness decreased with an increase in the molecular weight of PCDL. In addition, the coatings exhibited excellent thermal stability. With an increase of molecular weight, the Tg modulus and strength greatly decreased, whereas elongation at break increased relatively. These superior properties of the novel polymers could be attributed to the unique “hard core, flexible arm” structures.

Experimental Section

Materials

Hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) were provided by Dongrui Chemical Materials Co., Ltd. (Wuxi). Asahi Kasei Corporation (Japan) provided the PCDLs (molecular weight (Mn) = 2000, 1000, and 500 g/mol, named PCDL2000, PCDL1000, and PCDL500, respectively). Dibutyltin dilaurate (DBTDL) and hydroquinone were purchased from Aladdin (Shanghai, China); Tokyo Kasei Kogyo Co., Ltd supplied the hydroxyethyl methacrylate (HEMA); N,N-dimethylformamide (DMF) was supplied by Sinopharm Chemical Reagent, China; Irgacure 184 was kindly obtained from QiangLi New-Material Stock Co., Ltd (Changzhou, China); trifunctional acrylate (CD9051NS) was obtained from Sartomer Co., Ltd; and the hexakis[p-(hydroxymethyl)phenoxy]cyclotriphosphazene (H6) were synthesized according to the literature protocol.[24−26]

Synthesis of Monomer

Synthesis of Hexakis[p-(aldehydephenoxy)phenoxy]cyclotriphosphazene (HAPCP)

Refer to the method reported in the previous literature,[24] the synthetic route of the HAPCP was seen in Scheme . First, the thermometer and condenser were fitted onto a 250 mL three-necked flask and placed in a magnetic stirrer. In the flask, parahydroxybenzaldehyde (14.76 g, 0.12 mol), 50 mL of tetrahydrofuran (THF), and 20 mL of triethylamine were placed; the temperature rose to 65 °C. Then, dissolved phosphonitrilic chloride trimer (6.95 g, 0.02 mol) with 40 mL THF was added dropwise in a constant pressure drop funnel, stirred, and refluxed for 24 h. Bright yellow solution was obtained by filtered, and then the solvent was removed. Finally, the product was obtained and washed three times with ethanol and recrystallized with ethyl acetate, dried 12 h at a vacuum of 45 °C; pure white needle-like solid products were obtained. Figure S1 presents the 1H NMR spectra of HAPCP.

Scheme 1

Synthetic Route of the Monomer (HAPCP and H6)

Synthesis of H6

The synthetic route of the H6 was as follows (Scheme ). The HAPCP (4.31 g, 5.00 mmol), 30 mL of THF, and 15 mL of methanol were placed in a three-necked flask and stirred for 30 min. Then, 2 g of sodium borohydride was dispensed into the flask under an ice water bath and stirred for 24 h. The solvent was removed by rotary evaporation, and the product was washed with deionized water, recrystallized from ethanol immediately and finally dried at 45 °C to obtain the white product. Figure S2 presents the 1H NMR spectra of H6.

Preparation of UV-Cured Self-Healing PU

Scheme shows the synthesis of the self-healing PU. Two series of oligomers were prepared. First, 3.36 g of HDI or 4.67 g of IPDI and catalyst DBTDL were placed into a 250 mL four-necked flask equipped with a mechanical stirrer, thermometer, and condenser. After the temperature rose to 50 °C, 0.01 mol of PCDL (Mn = 500, 1000, 2000 g/mol) was added dropwise in half an hour, and then the reaction was stirred evenly at 50 °C for 2 h. Subsequently, dropwise added 1.30 g of HEMA was mixed with hydroquinone for 0.5 h under reflux conditions and reacted with the previous product s for 3 h to form the NCO-terminated prepolymer. Finally, the reaction was increased to 70 °C and the H6 dissolved in DMF was added to the flask for 7 h reaction until the reaction was complete. That is, the characteristic peak of NCO (2267 cm–1) disappears completely on the infrared spectrum (FTIR). The final oligomer was named IPH or HPH. In IPH, and HPH, the first letter denotes the type of isocyanate (IPDI or HDI), the “x” denotes the molecular weight of PCDL, and the “y” denotes the type of cyclotriphosphazene monomer.

Scheme 2

Synthetic Route of UV-Cured Self-Healing PU (IPH or HPH)

Preparation of UV-Cured Self-Healing Coatings

The aluminum plate was cleaned with acetone before being used as the coating substrate. IPH or HPH was used as the matrix resin and Irgacure 184 (3 wt %) as the photoinitiator, mixed with trifunctional acrylate (CD9051NS, 2 wt %) to prepare UV-cured coating. A 120-thick wet film was daubed to the substrate with the doctor blade. The wet film was prebaked at room temperature for 2 h and immediately placed on a hot plate until the solvent was completely removed. Finally, each sample was cured under F300 UVA lamp (the light intensity measured by the German-designed UV INT 140 was 120 mW/cm2) for 60 s to obtain a series of cured coatings.

Characterizations of UV-Cured Self-Healing PU and Coatings

The AVANCE III HD 400 MHz (Bruker) was used to measure the nuclear magnetic resonance spectra (1H NMR) of IPH or HPH. The samples to be tested were dissolved in dimethylsulfoxide and tested as tetramethylsilane internal standard at four temperatures (25, 50, 75, and 100 °C). A Thermo Nicolet Nexus FTIR spectrometer was used to measure FTIR spectra of IPH or HPH. The wavenumber ranges from 500 to 4000 cm–1. The molecular weight of the polymer was determined by gel permeation chromatograph (the eluent: THF, flow rate: 1.0 mL/min). The rheological properties of the polymer were investigated by DHR-2 Discovery rheometer (TA Instruments). All samples were measured in parallel plate system dynamic mode (angular frequency ranging: 0.01–200 rad/s, plate diameters: 25 mm). A 120 μm thick coating was coated on the substrate, measuring its gloss at 60° reflective angle. The hardness of the pendulum was measured with PH-5858, BYK, Germany, using the ASTM D 4366 test mode, in seconds as the hardness unit. According to the standard crosshatch adhesion test (ASTM D 3359), the adhesion of coatings was tested (QFH, Tianjin Jingke Material Testing Co. Ltd.). The atomic force microscope (AFM MuLtimode) from the Bruker Company was used to observe self-healing properties of coatings; first, the surface of the coating was scratched with a scalpel, followed by heat treatment of the damaged surface with a heat gun, and the performance before and after the coating damage was taken by AFM (hot air gun was 5 cm away from the surface, heating time was 1 min, temperature was 180 °C). Using ultra-deep-field 3D microscope (KH-8700; Hirox) from the Keyence Company, the self-healing performance in different healing times was observed. The healing process was carried out at 100 °C on a hot plate (SET) from Shenzhen Fan and hang Co. Ltd. Thermal stability were characterized using the TGA 1100SF instrument by heating the samples from 30 to 600 °C; the heating rate was 20 °C/min. Dynamic mechanical analysis (DMA) was measured by the DMA Q800 (TA Instruments). Temperature range was from −50 to 140 °C, heating rate was 3 °C/min, and test frequency was 1 Hz. The tensile tests results were examined by the Instron 5967 at the room temperature; the average stretching rate was 20 mm/min (six samples were 15 mm in length and 4 mm in thickness).