Development of Catalyst-Free Self-Healing Biobased UV-Curable Coatings via Maleate Monoester Transesterification

Abstract

Developing environmentally friendly UV-curable polymers with multi-functionality is very significant for sustainable development and environmental protection. In this work, a novel tung-oil-based UV-curable oligomer (TOMAH) was synthesized by Diels–Alder and ring-opening reactions via microwave technology. Subsequently, catalyst-free self-healing UV-curable materials based on a maleate monoester transesterification (MMETER) were developed by co-photopolymerization of TOMAH and hydroxyethyl methacrylate (HEMA). The obtained UV-cured materials possessed a high glass transition temperature (T_g > 81 °C), excellent adhesion (grade 1), and flexibility (2 mm). Particularly, the outstanding photopolymerization activity of the UV-curable resins was proved by UV-curing kinetics. In addition, dynamic transesterifications occurred without an external catalyst at a moderate temperature, resulting in good self-healing properties (with a scratch-repair efficiency of 78.6–93.3%) and shape-memory properties for the obtained UV-cured materials. This work combines the multiple advantages of biomass raw material, microwave synthesis technology, UV-curing method, and multifunctional polymers, thus providing an innovative strategy to fabricate sustainable and intelligent coatings.

1. Introduction

Ultraviolet (UV)-curing technology is known as a new technology for green industry in the 21st century, with the advantages of “5E” (efficient, enabling, economic, energy saving, and environmental protection); it is widely used in coatings [1,2], ink [3], adhesives [4,5], 3D printing [6,7], and other fields. With the improvement of the awareness of low energy consumption and environmental protection, the traditional solvent-based coatings, adhesives, and other markets are gradually being occupied by UV-cured products. The reported UV-cured resins are mainly derived from non-renewable fossil resources and are thus detrimental to sustainable development and aggravate the energy crisis. In addition, due to the irreversibility of the chemical bond, the UV-cured materials are difficult to recycle and reprocess once they are scratched or damaged. The traditional method of handling the damaged resins is mainly incineration or sending them to the landfill, which not only wastes resources, but also aggravates environmental pollution. Therefore, there is an urgent need to develop biobased UV-curable materials which can self-heal and be recycled.

In order to cope with the inevitable damage of materials in application, the US military developed smart materials that can heal defects in 1986 [8], and the specific concept of self-healing was proposed by Dry in the early 1990s [9]. According to the self-healing mechanism, these materials mainly included extrinsic and intrinsic categories. The former is filled with micro-nanostructured healing agents in the polymer matrix. Once the material is broken, the healing agents flow out and repair the damaged part in situ [10]. Zhang et al. [11] synthesized polyurea–formaldehyde microcapsules under alkaline conditions and then embedded them into epoxy coatings, under acidic conditions, to form self-healing coatings. Nevertheless, this method has a narrow range of applications; for example, it is not suitable for thin-film coatings. The intrinsic category is based on dynamic bond breaking and recombination; these materials can achieve multiple self-repairing. In addition, the dynamic bonds can be divided into dynamic non-covalent bonds (such as ionic bonds, hydrogen bonds, and coordination bonds [12,13,14,15]) and dynamic covalent bonds (such as ester bonds, disulfide bonds, boron-based bonds, and imine bonds [16,17,18,19,20,21,22,23,24]). Compared with dynamic non-covalent bonds, dynamic covalent bonds have the advantages of a higher bond energy, stronger stability, solvent resistance, and being less influenced by environmental polarity.

UV-curable materials based on a dynamic covalent bond possess both the advantages of UV-curing technology and dynamic covalent bonds. Many dynamic covalent bonds, including ester bonds, disulfide bonds, sterically hindered urea bonds, etc., have been introduced into the construction of UV-curable self-healing materials. Among them, self-healing polymers based on dynamic transesterification reactions (DTERs), also called vitrimers, can undergo rapid DTERs under the condition of a high temperature and with a catalyst [25]. Lu et al. [26] developed a 3D-printable photosensitive resin, using methacrylated cellulose and rosin-based derived monomers as raw materials. Based on the combined effect of DTERs and hydrogen bonding, the damaged material can be fully repaired after photo-curing and heat treatment. Wang et al. [27] explored two kinds of UV-curable prepolymers by reacting itaconic acid and furoic acid with glycidyl methacrylate, sequentially. The prepolymers not only have good mechanical and thermal properties; they also can easily self-heal and be recycled. Although dynamic covalent polymers based on DTERs have been developed rapidly, a series of shortcomings remain to be overcome. For example, the occurrence of DTERs usually requires the assistance of an expensive external catalyst because they are easily deactivated during the heating process and corrode the substrates when used in the coating field [28]. Although some reports based on DTERs without a catalyst were presented, the mechanical properties of the resulting materials were generally low, and the recovery effect was unsatisfactory [25,29]. Recently, Du Prez and coworkers [30] developed phthalate monoester transesterification (PMETER) as a novel chemistry platform for covalent adaptable networks. The ester exchange reactions can happened quickly between the monoesters by a dissociative mechanism that is caused by internal catalysis of the free carboxylic acid, which reversibly forms an activated phthalic anhydride intermediate.

Tung oil (TO) is a kind of vegetable oil with the characteristics of rapid drying, high temperature resistance, excellent corrosion resistance, and good water resistance; thus, it has been widely used in coatings, paints, printing, agricultural machinery, electronic industry, and so on [29]. The low viscosity and fast reaction kinetics of TO, as well as the excellent properties of the resulting thermoset resins, make TO and its derivatives perfect candidates for self-healing UV-curable materials.

Herein, we report the development of catalyst-free self-healing TO-based UV-curable coatings. Firstly, TO reacted with maleic anhydride (MA) to generate tung oil maleate (TOMA), and then the TOMA reacted with 2-hydroxyethyl methacrylate (HEMA) via ring-opening modification to obtain a tung-oil-based prepolymer (TOMAH). Subsequently, the TOMAH was co-photopolymerized with HEMA to prepare multifunctional UV-cured coatings. It should be stated that the free carboxylic acid on TOMAH can reversibly form an activated anhydride intermediate when heating the cured TOMAH/HEMA materials, leading to fast transesterification. This maleate monoester transesterification (MMETER) is similar to the report PMETER. Moreover, with the aid of microwave technology, the synthesis of the TOMAH prepolymer took only 40 min. Finally, the ultimate properties of the cured resins were explored in detail, including the general (physical, thermal, mechanical, and coating properties) and dynamic performance (repairing, shape memory, malleable, and recyclable properties).

2. Materials and Methods

2.1. Materials

Tung oil (TO) was supplied by Jiangsu Donghu Bioenergy Plant Plantation Co., Ltd. (Yancheng, China); its color was brown, and its density at 25 °C was between 0.935 and 0.940 g/cm³. Maleic anhydride (MA, 99%), hydroxyethyl methacrylate (HEMA, ≥96%), and 4-methoxy-phenol (MEHQ, ≥99.5%) were purchased from Aladdin Reagent China. Toluene (AR) and 4-Dimethylaminopyridine (DMAP, 98%) were bought from Jiuding Chemistry Co., Ltd. (Shanghai, China). Darocur 1173 initiator (≥99%) was provided by Tianjin Chemical Reagent Institute Co., Ltd. (Tianjin, China). Sodium magnesium anhydrous (Mg₂SO₄) and dichloromethane (AR) were provided by Maclin Co., Ltd. (Shanghai, China).

2.2. Synthesis of TOMA

TO was maleated on a Discover SP microwave synthesizer (CEM Corporation, Mathews, NC, USA). First, 7.00 g of TO and 2.36 g of MA were added to a 35 mL standard glass reaction tube at 160 °C for 15 min. Then the reaction products were moved to a 20 mL round flask equipped with two ball tubes and evacuated at 110 °C for 120 min by a B-585 Kugelrohr glass oven (Büchi Corporation, Flawil, Switzerland). The unreacted MA was collected by a cooling device under the ball tubes. Finally, a light brown and viscous liquid TOMA was obtained.

2.3. Synthesis of TOMAH

Similarly, TOMAH was synthesized on the microwave synthesizer. A total of 3.0 g of TOMA, 1.0 g of HEMA, 0.04 g of DMAP (1 wt.%), 0.002 g of MEHQ (0.5 wt.%), and 3 mL of toluene were added to a 35 mL standard glass reaction tube at 90 °C for 30 min. Subsequently, the crude product was evaporated via rotary evaporation to remove toluene, and then it was dissolved in dichloromethane. Then the mixture was washed three times with a hot 10 wt.% NaCl water solution and dried by Mg₂SO₄ for 12 h. The solvent was then removed by rotary evaporation. At last, the product was dried at 50 °C for 5 h under vacuum, and a brown viscous liquid product was obtained. The synthesis route of TOMAH is shown in Scheme 1.

2.4. Curing of TOMAH Resins

The formulations of the TOMAH samples are displayed in

Table 1. Compositions of TOMAH/H samples.

Samples	TOMAH (g)	HEMA (g)	D1173 (g)
TOMAH/H10%	9	1	0.3
TOMAH/H20%	8	2	0.3
TOMAH/H30%	7	3	0.3

. TOMAH, HEMA, and Darocur 1173 (D1173) were successively added to a beaker, stirred at room temperature for 30 min, and degassed under vacuum for 4 h. For example, the TOMAH/H10% signified that the formula contained 90 wt.% prepolymer TOMAH and 10 wt.% HEMA. The resulting samples were cast into homemade polytetrafluoroethylene molds with different shapes and sizes, or they were painted on polished tinplate sheets, using a film maker. Finally, the cast or painted samples were light cured, using an Intelli-Ray 400 W UV-light-curing microprocessor from Uvitron International, West Springfield, MA, USA. All of these samples were exposed to UV light at wavelengths of 320–390 nm and an exposure intensity of 100 mW/cm² for 1200 s.

2.5. Characterization

2.5.1. Fourier-Transformed Infrared Spectroscopy (FTIR)

FTIR spectroscopy was performed on a Nicolet iS10 infrared spectrometer from Thermo-Fisher Cooperation (Waltham, MA, USA), using the attenuated total reflectance method. The background was collected before any measurements were made. All measurements were performed in the spectral range from 500 to 4000 cm⁻¹, with a resolution of 4 cm⁻¹. The surfaces of the samples to be measured were scanned against the IR transmissive crystal surface, and the IR spectra of the surface of the sample to be measured were obtained by scanning the collected scan curve and subtracted from the baseline for correction. Bruker spectroscopy software was used for the data analysis.

2.5.2. Nuclear Magnetic Resonance (NMR)

Using the deuterated chloroform (CDCl₃) as a solvent, we carried out ¹H NMR spectroscopy on a DRX-300 advection NMR spectroscopy from Germany Bruker Corporation (Karlsruhe).

2.5.3. Gel Content (C_gel) and Biobased Contents (C_bio)

The C_gel values of the samples were obtained by Soxhlet extraction. The cured samples were first dried in an oven at 60 °C for 24 h, weighed accurately (denoted m₀), and then extracted with acetone for 24 h, followed by drying the extracted samples in a vacuum oven at 60 °C for 24 h; then they were weighed again (denoted m₁). The C_gel values can be determined simply by using the formula C_gel (%) = (m₁/m₀) × 100%. For accuracy, at least three samples should be tested to obtain the average data.

The United States Department of Agriculture (USDA) defines the biobased content of a product as “the amount of biobased carbon in a material or product as a percentage of the weight (mass) of the total organic carbon in the product” [31]. Based on this definition, the biobased content of the raw materials TO, TOMA, and HEMA was 100%, 0%, and 0%, respectively.

2.5.4. UV-Kinetics Behavior

A UV Nicolet 5700 spectrometer bought from USA-Nicolet Instruments (Madison, WI, USA) was used to study the light-curing kinetics of TOMAH/H resins. These resins were light-cured under UV irradiation with an intensity of 100 W/cm². The double-bond (C=C) conversion rate was determined by monitoring the peak intensity at approximately 810 cm⁻¹ [32]. The polymerization rates of the UV-curable TOMAH/H resins can be obtained by taking the first-order derivative of the double-bond conversion rate with respect to time, t.

2.5.5. Dynamic Mechanical Analysis (DMA)

DMA tests were performed on cured samples that were 40 mm × 6 mm × 1 mm in size, at 1 Hz, in stretch mode, using a Q800 solid state analyzer (TA Corporation, Los Angeles, CA, USA). The test temperature increased from −50 to 200 °C, at a rate of 5 °C/min.

2.5.6. Thermal Gravimetric Analysis (TGA)

The TGA tests were performed on an STA 409PC thermogravimeter from Netzsch, city, Germany. The tested samples were first ground to a powder, and approximately 10 mg of the sample was taken and heated at a rate of 20 °C/min, under N₂ conditions, to monitor the thermal stability in the range of 35 to 600 °C.

2.5.7. Stress Relaxation Test

A Q800 solids analyzer (TA Corporation, Alda, NE, USA) with the iso-stress value set at 0.5 MPa was used to obtain the stress relaxation curves. All of the samples were sized 50 mm × 6 mm × 1 mm in advance.

2.5.8. Mechanical Properties

The mechanical properties of the UV-cured samples were examined by a SUNS7 CMT-4304 general-purpose apparatus (Suns Instrument Company, Shenzhen, China) in accordance with the ASTM D5402-06 standard. The sample size was 100 mm × 10 mm × 1 mm, and each sample was tested 5 times to obtain the average value; the tensile rate of the samples was 5 mm/min.

2.5.9. Coating Properties

An adhesion test was carried out on the adhesion test machine of Tianjin Shiboweiye Glass Instrument Co., Ltd. (Tianjin, China), in accordance with GB 1720-79 (89). Adhesion is rated from 1 to 7 (1 is the best). The pencil hardness of the cured coating was tested on a QHQ-A pencil-hardness tester from the Tianjin Littengda Instrument company, according to the GB/T 6739-2006 standard. Hardness is in the range from 6B to 6H (the grade 6H is the hardest). According to the GB/T 1731-93 standard, the flexibility test was carried out on the QTY-32 paint film cylindrical bending machine from the Tianjin Littenda Instrument company (China). Flexibility ranges from 2 nm to 32 mm (preferably 2 mm). Solvent-resistance tests were performed according to ASTM D5402-06, using toluene, ethanol, water, and acetone as solvents.

2.5.10. Repairing Properties

The self-healing properties of this UV-curable material were evaluated by estimating the recovery of scratches, using a microscope from America Leica Corporation (Sioux Falls, SD, USA). A thin blade was used to scratch the UV-curable film (on a thin glass sheet) with a thickness of approximately 200 µm, and the width of the scratch (noted as D₁) was measured with a microscope. The sample was heated at 180 °C for a period of time, and the crack width (noted D₂) was measured again. The repairing rate (Rr) was calculated by using Equation (1):

$$R_{\mathrm{r}}=\frac{D_1 - D_2}{D_1}\times 100\%$$

(1)

2.5.11. Shape Memory Properties

The DMA test was used to monitor the shape-memory properties of the obtained UV-cured material. In general, rectangular specimens of 40 mm × 6 mm × 1 mm were heated to 130 °C at 5 °C/min, and then a constant strain of 5% was applied to induce shape change. The samples were then cooled to 30 °C at 5 °C/min. After that, the external stress was removed at once. The whole process was repeated four times. The following equations were used to calculate the shape-fixation rate (R_sf) and shape-recovery rate (R_sr):

$$R_{\mathrm{sf}}=\frac{{\mathsf{\epsilon }}_{\mathrm{unload}}}{ {\epsilon }_{\mathrm{load}}}\times 100\%$$

(2)

$$R_{\mathrm{sr}}=\frac{{\epsilon }_{\mathrm{unload}}-{\epsilon }_{\mathrm{rec}}}{{\epsilon }_{\mathrm{unload}}}\times 100\%$$

(3)

where ε_unload, ε_load, and ε_rec represent the fixed strain of each cycle, initial strain upon loading, and recovered strain, respectively.

2.5.12. Recyclable Properties

The samples cured by UV light were crushed into powder form by a crusher, put into a metal mold of 60 mm × 60 mm × 1 mm, and then moved into a hot press, where the powder was molded at 180 °C and 15 MPa for 60 min and cooled to room temperature. Finally, five rectangular samples with a size of about 60 mm × 10 mm × 1 mm were cut from the recovered material and tested for tensile properties by a universal testing machine.

3. Results

3.1. Characterization of TOMA and TOMAH

The FTIR spectra of TO, TOMA and TOMAH are illustrated in Figure 1a. The spectrum of TO showed carbonyl groups of 1742 cm⁻¹, C-O-C antisymmetric extensional vibration of 1155 cm⁻¹, and C=C vibration of conjugated triene of 990 cm⁻¹. Compared with the spectrum of TO, new peaks at 1843 cm⁻¹ and 1778 cm⁻¹ appeared in the spectrum of TOMA, which were related to the C=O groups in MA. Furthermore, the peak at 990 cm⁻¹ of conjugated triene of TO disappeared, indicating that MA was successfully reacted with the conjugated triene of TO [33]. In the TOMAH spectrum, obviously, the peaks at 1843 cm⁻¹ and 1778 cm⁻¹ corresponding to the anhydride group vanished [34]. Meanwhile, the peak at 1719 cm⁻¹ was enhanced, and this contributed to the formation of carboxyl groups by the ring-opening reaction of TOMA with HEMA.

The ¹H NMR spectra of TO, TOMA, and TOMAH were shown in Figure 1b. For TO, a vinyl proton peak representing C=C on the triglyceride chain appeared at 5.2 to 6.4 ppm. However, the signals of C=C bond could hardly be seen at 6.00 to 6.40 ppm for TOMA, suggesting that the C=C on the triglyceride chain was reacted with MA. In particular, new proton peaks from 2.9 to 3.50 ppm belonged to the generated -CH at the junction of TO and MA by the Diels–Alder reaction [35]. In the TOMAH spectrum, the weak peak at 7.7 ppm was attributed to the carboxyl group generated by the ring-opening reaction, and the peaks at 5.6–6.2 ppm were attributed to vinyl protons introduced from HEMA. The peaks at 0.88 ppm and 1.9 ppm were assigned to the terminal -CH₃ on TO triglyceride and the -CH₃ of HEMA, respectively. Methyl proton peaks on TO triglycerides are often used as reference for calculating other proton peaks because of their constant numbers throughout the process. The C=C functionality (N_C=C) of TOMAH can thus be calculated by using Equation (4) [32,36]:

$$N_{\mathrm{C}=\mathrm{C}}=\frac{A_{5.6–6.2\mathrm{ppm}/2}}{A_{0.9\mathrm{ppm}/9}}=\frac{{9A}_{5.6–6.2\mathrm{ppm}}}{{2A}_{0.9\mathrm{ppm}}}$$

(4)

The N_C=C of TOMAH was 3.11 per triglyceride.

3.2. UV-Curing Kinetics

The UV-curing kinetics of the TOMAH/H resins was explored by real-time IR (RT-IR). Figure 2 and

Table 2. UV-curing parameters and physical properties of the TOMAH/H resins.

Samples	αfa (%)	Rpb (s−1)	Cgelc (%)	Cbiod (%)
TOMAH/H10%	86.7	0.196	94.1 ± 0.2	46.5
TOMAH/H20%	88.2	0.223	94.6 ± 0.1	41.4
TOMAH/H30%	90.8	0.228	94.8 ± 0.2	36.2

^a Final C=C conversion. ^b Maximum C=C conversion rate. ^c Gel content. ^d Biobased content.

presents the C=C conversions (α_f) and polymerization rates (R_p) of the TOMAH/H resins, respectively. All the final α_f values of TOMAH/H resins surpassed 86% after 20 min and achieved the highest R_p within about 8 s, suggesting the outstanding photopolymerization activity of the obtained resins. As the dosage of HEMA was raised from 10% to 30%, the α_f and maximum R_p rose from 86.7% to 90.8% and from 0.196 to 0.228 s⁻¹, respectively. The reason lies in the fact that the higher content of HEMA can reduce the viscosity of the resultant resins and facilitate the movement of the free radicals [37,38].

3.3. Gel Contents and Biobased Contents

As can be seen from the Table 2, all samples exhibited high C_gel values (above 94%), indicating that the UV-cured TOMAH/H materials had high degree of crosslinking. Notably, the C_gel increased with the addition of HEMA, mainly because of the improved C=C conversions, which was consistent with the above results of the RT-IR.

Based on the definition of C_bio, i.e., the weight percent of biobased carbon in overall organic carbon, the C_bio values of TOMA, HEMA, and Darocur 1173 were 82.6%, 0%, and 0%, respectively, while those of the TOMAH/H materials fell into the range between 36.2% and 46.5%, as exhibited in Table 2, which were also obviously in the higher ranks of self-healing UV-cured materials reported [23,39].

3.4. Thermal, Mechanical and Coating Properties of the TOMAH/H Resins

The changes of storage modulus (E′) and loss factor (tan δ) with temperature for the cured TOMAH/H samples are presented in Figure 3a,b, and the related data are summarized in

Table 3. Thermal properties of the cured TOMAH/H materials.

Samples	E′25a (MPa)	Tgb (°C)	E′Tg+50c (MPa)	νed (mol/m3)	T5e (°C)	Tpf (°C)	wcharg (%)
TOMAH/H10%	1505.1	81.1	18.6	1844	303.9	429.9	2.51
TOMAH/H20%	1653.7	86.8	17.7	1731	301.4	433.2	2.30
TOMAH/H30%	1756.2	90.0	16.1	1563	296.4	434.0	3.17

^a Storage modulus at 25 °C. ^b Glass transition temperature. ^c Storage modulus at T_g + 50 °C. ^d Crosslink density. ^e 5% weight loss temperature. ^f Peak temperature at the curves of weight loss rate. ^g Char yield.

. The glass transition temperature (T_g) values were gained from the peaks of tan δ curves, and the UV-cured materials manifested high T_g values (>80 °C) [36]. With the dosage of HEMA climbing from 10% to 30%, the E′ at 25 °C (E′₂₅) and the T_g of the TOMAH/H materials gradually increased from 1505.1 to 1756.2 MPa and from 81.1 to 90.0 °C, respectively. The crosslink density (ν_e) of the specimens was calculated by the following formulation [36]:

$${\nu }_{\mathrm{e}}=\frac{E\prime }{3\mathit{RT}}$$

(5)

where E’ is the storage modulus of a material in the region of rubbery plateau (in this work, E’ at T_g + 50 °C was used to determine ν_e), R is the universal gas constant, and T is the absolute temperature.

With the increase of HEMA content, the ν_e of the resulting material also decreased from 1844 to 1563 mol/m³, which is ascribed to the use of a monofunctional monomer, which increases the effective molar mass between crosslinked sites. V_e should be “ν_e”

Figure 3c showed the TGA curves of UV-cured TOMAH/H materials and their derivatives, and the respective thermal properties for the main peaks were given in Table 3. All the samples were stable below 300 °C, while they all decomposed quickly in the range of 350–430 °C, indicating the destruction of crosslinked networks in this situation. As the dosage of HEMA rose from 10% to 30%, the 5% weight-loss temperature (T₅) decreased from 303.9 to 296.4 °C, probably because there was a greater number of ester bonds in the systems from HEMA and they were unstable at a high temperature. All the UV-cured samples showed high maximum thermal decomposition temperature (T_p) from 429.9 to 434.0 °C, symbolizing excellent thermal stability of the obtained systems.

Typical stress–strain curves were shown in Figure 3d, and the detailed data are presented in

Table 4. Tensile properties of the UV-cured TOMAH/H samples.

Samples	σa (MPa)	E b (MPa)	ε c (%)
TOMAH/H10%	26.7 ± 0.3	243.4 ± 9.1	15.5 ± 0.9
TOMAH/H20%	28.0 ± 0.1	280.5 ± 28.5	21.1 ± 0.4
TOMAH/H30%	31.3 ± 1.1	291.0 ± 10.0	16.3 ± 2.3

^a Tensile strength. ^b Young’s modulus. ^c Tensile breaking strain.

. As the dosage of HEMA rose from 10% to 30%, the tensile strength and modulus of the samples increased from 26.7 to 31.3 MPa and from 243.4 to 291.0 MPa, respectively, and this is ascribed to the increased methyl steric hindrance structure. As the concentration of HEMA was raised from 10% to 30%, the elongation at break increased firstly and then reduced. The decreasing crosslink density and increasing content of the rigid methyl group from HEMA may contribute to the variation of elongation at break.

The coating properties of the UV-cured TOMAH/H samples are presented in

Table 5. Coating properties of the UV-cured TOMAH/H samples.

Samples	Adhesion (Grade)	Pencil Hardness	Flexibility (mm)	Solvent Resistance (Rubs)
				Water	Ethanol	Acetone	Toluene
TOMAH/H10%	1	HB	2	>200	>200	>200	>200
TOMAH/H20%	1	H	2	>200	>200	>200	>200
TOMAH/H30%	1	2H	2	>200	>200	>200	>200

. It was noticed that the adhesion and flexibility for the samples containing different percentages of HEMA both remained at the best level. The excellent adhesion is attributed to the existence of lots of hydroxyl groups, and the outstanding flexibility is ascribed to the large number of flexible fatty acid chains of TO [31,40,41]. In contrast, the pencil hardness of the resulting specimen increased from HB to 2H due to the increasing content of methyl steric structures of HEMA. In addition, after being wiped back and forth with different solvents (water, ethanol, acetone, and toluene) two hundred times, all coated surfaces were intact, indicating their excellent solvent resistance.

Figure 4 demonstrated the stress-relaxation behaviors associated with the cured TOMAH/H material. The relaxation rate is usually expressed in terms of the characteristic relaxation time (τ*), which is defined as the time required to relax to 1/e of its initial modulus. Figure 4a demonstrated that all samples exhibited a significant stress-relaxation behavior at 180 °C. The stress-relaxation phenomenon of the UV-cured TOMAH/H material provided direct evidence for the MMETERs in the systems. With the increase of HEMA content from 10% to 30%, the τ* value increased from 829 s to 2157 s, which is possibly ascribed to the growing content of methyl steric structures from HEMA. Taking the TOMAH/H20% sample as an example to test its stress relaxation time at different temperatures, as shown in Figure 4b, all samples exhibited significant stress-relaxation behavior at 160 to 200 °C and presented faster relaxation rates at higher temperatures, which indicated that the exchange rate of DTERs in the network was faster, while the temperature increased.

More significantly, the efficient topology change of cured TOMAH network was achieved without a catalyst. The -OH of the crosslinked network not only acts as a reactive group, but it also acts as a catalyst for DTERs at high temperatures. Moreover, the activation energy (E_a) was calculated by using the following equation [42,43]:

$$\ln \tau *=\frac{E\mathrm{a}}{RT}-\ln A$$

(6)

where T is the absolute temperature, A is a pre-exponential factor, and R is the universal gas constant. Accordingly, the E_a of cured TOMAH/H20% was 68.63 kJ mol⁻¹, which was clearly lower than the values of the reported materials based on DTERs with external catalysts (usually in the range of 80 to 150 kJ mol⁻¹) [25,44].

3.5. Self-Healing Properties

The scratch-repairing test was performed to verify the self-healing performance of the UV-cured TOMAH/H films, as shown in Figure 5a. The cured film was scratched with a razor blade to form a uniform crack on the surface, and then it was repaired at 150 °C for 30 min in a stress-free state. The TOMAH/H films exhibited a remarkable self-healing performance, which reached a self-healing efficiency of more than 78% within 30 min. Especially for the films with 20% and 30% HEMA, the self-healing efficiency was high, at 93.3% and 92.3%, respectively, and this can be attributed to the fact that the primary hydroxyl groups of HEMA participated in DTERs under heating. However, when the HEMA content increased from 20% to 30%, the self-repairing efficiency at 30 min decreased slightly, possibly due to the increasing content of sterically hindered structures of the methyl groups [45]. Furthermore, the self-healing mechanism of the obtained material is illustrated in Figure 5b. At high temperatures, the carboxylic acid on the phthalic acid side of the reaction acts as a catalyst to accelerate the dynamic ester exchange reaction, avoiding the use of a highly active external catalyst [30]. In the experiment, we found that when the content of diluent HEMA continued to increase, the self-repair efficiency of the coating first increased and then decreased, and when the content of HEMA was 20%, the self-repair effect of the coating was optimal, so we only increased the percentage of HEMA to 30%.

3.6. Shape Memory and Recyclable Properties

The TOMAH/H materials are rich in triglyceride chains and dynamic ester bonds, suggesting that the UV-cured TOMAH/H resins should possess shape-memory functions and plasticity [46]. Herein, the TOMAH/H20% sample was chosen to investigate the shape-memory properties on the basis of its relatively prominent thermal and mechanical properties. First, the shape-memory function was qualitatively verified by a simple twist/bend-recovery experiment. The related process is shown in Figure 6a. The strip sample (i) was heated at 120 °C for 10 min, quickly twisted into a W-shape (ii), and then immediately immersed in cold water at 25 °C to fix this shape. After that, the twisted sample was then heated to 120 °C and fully recovered to the initial shape (iii). Subsequently, the recovered sample was wrapped with tin foil and fixed into an S-shape, heated at 150 °C for 30 min, and then quickly fixed in cold water at 25 °C. The sample was formed into a strong S-shape (iv). Then the sample was heated to 150 °C under the action of external force, and the S-shaped sample could return to the initial shape (v). When the external force was removed, the samples quickly recovered to the S-shape (iv) at 150 °C, and it was able to hang a 200 g weight vertically without breaking, thus indicating that the sample had good shape-memory properties after remodeling.

In addition, the shape-memory properties of UV-cured TOMAH/H samples were quantitatively analyzed by DMA. The sequential double-shape-memory cycles of the TOMAH/H materials are displayed in Figure 6b, and the relevant data are shown in

Table 6. Shape fixation and recovery ratios (R_sf and R_sr) in a continuous dual-shape-memory process.

Cycle	Rsf (%)	Rsr (%)
Cycle 1	98.9	65.8
Cycle 2	98.9	63.8
Cycle3	99.1	62.3
Cycle 4	99.0	66.9

. The shape-fixation ratios (R_sf) were over 98%, and the shape-recovery ratios (R_sr) of the samples were 62.3–66.9% after four cycles. The higher R_sf values are mainly due to the great difference between the glassy and rubbery moduli, while the relatively lower R_sr values are probably because of the effect of the dynamic bond exchange.

The TOMAH/H20% sample was used to carry out the recycling experiment, as displayed in Figure 6c. The stress–strain curves for various specimens are illustrated in Figure 6d, and the respective mechanical behaviors are summarized in

Table 7. Mechanical properties of the original and recycled materials.

Samples	σa (MPa)	Re d (%)	Eb (MPa)	Re d (%)	ε c (%)	Re d (%)
Origin	28.0 ± 0.1	/	280.5 ± 28.5	/	21.1 ± 0.4	/
Cycle1	9.61 ± 0.7	34.3	92.9 ± 15.3	33.1	11.7 ± 1.2	55.5
Cycle2	4.5 ± 0.3	16.1	57.9 ± 11.2	20.6	8.6 ± 0.3	40.8

^a Tensile strength. ^b Young’s modulus. ^c Tensile breaking strain. ^d Recycling efficiency for each kind of property.

. The cured rectangular splines were first ground into powder by a pulverizer and then placed under the conditions of 180 °C and 15 MPa for hot pressing. Eventually, the sample was cut into strips of 1 mm × 10 mm × 10 cm for the mechanical test. The tensile strength and elongation at break of the samples decreased significantly after recycling. The reason may be the severely damaged crosslinked structure of the material during the grinding process and the fact that the crosslinked network formed during the hot-pressing process further restricted the process of the PMETER.

4. Conclusions

A biobased UV-curable oligomer TOMAH was synthesized based on TO via two-step reactions, and then a catalyst-free self-healing UV-curable coating of TOMAH/H was developed based on a new type of DTERs, called MMETER. In detail, the systems exhibited a comprehensive performance: high C_gel (>94%), excellent α_f (90.8%), high thermal properties (T_g > 80 °C), strong tensile strength (up to 31.3 MPa), and superior coating properties (adhesion of 1 grade, pencil hardness of 2H, and flexibility of 2 mm). Moreover, the introduction of phthalate monoester transesterification without a catalyst endowed the TOMAH/H materials with good self-healing abilities (78.6–93.3%), shape memory, plasticity, and recyclability. Overall, this work developed a biobased UV-curable coating via a friendly and efficient manner and provided an innovative method for the fabrication of catalyst-free self-healing materials.