Preparation of High Self-Healing Diels–Alder (DA) Synthetic Resin and Its Influence on the Surface Coating Properties of Poplar Wood and Glass

Abstract

Self-healing coatings can replace conventional coatings and are capable of self-healing and continuing to protect the substrate after coating damage. In this study, two types of self-healing resins were synthesized as coatings: Type-A via Diels–Alder crosslinking of furfuryl-modified diglycidyl ether bisphenol A with bismaleimide, and Type-B through epoxy blending/curing to form a semi-interpenetrating network. FTIR and Raman spectroscopy confirmed the formation of Diels–Alder (DA) bonds, while GPC tests indicated incomplete monomer conversion. Both resins were applied to glass and wood substrates, with performance evaluated through TGA, colorimetry (ΔE), gloss analysis, and scratch-healing tests (120 °C/30 min). The results showed that Type-A resins had a higher healing efficiency (about 80% on glass substrates and 60% on wood substrates), while Type-B resins had a lower healing rate (about 65% on glass substrates and 55% on wood substrates). However, Type-B is more heat-resistant, has a slower decomposition rate between 300 and 400 °C, higher gloss retention, and less color difference (ΔE) between wood and glass substrates. The visible light transmission of Type-B (74.14%) is also significantly higher.

1. Introduction

The global shortage of fossil resources and the attendant environmental risks and price instability continue to drive innovative, multidisciplinary research into alternative materials. Research has shown that polymers with integrated features such as reusability, recyclability and self-healing have a great potential for waste containment and resource conservation, and their properties are a strategic fit with the goals of sustainable development [1,2,3]. Furthermore, excellent materials such as wood rely heavily on coatings to protect the base material [4,5,6]. Therefore, it is necessary to develop self-healing polymer materials, which are capable of healing physical or chemical damage and restoring their mechanical properties [7,8,9,10]. Self-healing polymers developed at this stage are categorized into exogenous self-healing and intrinsic self-healing polymers, with the former requiring pre-embedded healing agents, and the latter allowing for damage healing through the reversible chemical bonding of the polymer itself [11,12,13,14,15].

For exogenous self-healing polymers, the injury location does not allow for multiple healing at the same location after healing due to the absence of healing agents, whereas intrinsic self-healing materials do not change after healing and allow for multiple healing at the same location. Therefore, intrinsically self-healing polymers have received more attention in research [16,17,18,19,20,21]. In principle, the healing mechanism of most intrinsically self-healing materials is based on the breakage and reorganization of the reversible chemical bonds of the polymers under certain conditions, which makes the material mobile and heals the damage [22,23,24,25,26,27].

Epoxy resin is a common thermosetting material that is widely used in various fields for its unique physical and chemical properties [28,29,30,31]. As a component of composite materials, coatings, and adhesives, the epoxy resin is not only used in transportation and military applications, but also widely used in industry, agriculture, construction, and building materials. And as a smart material, shape memory epoxy resin with self-healing function will simplify the structure of components and improve the stability of the system [32,33,34,35,36]. Recently, various epoxy resins have been developed for both general-purpose applications as well as specific areas such as thermal conductive materials because of their strong adhesion properties, molecular designability realizability, etc. [37,38,39,40,41,42]. Also, a lot of research has been conducted on epoxy composites, including epoxy-based materials reinforced by reactive or non-reactive block copolymers [43,44]. The research hotspots lie in the development of novel curing agents, design of particle-reinforced nanocomposites, and improvement of the overall properties of epoxy resins [45,46].

Dynamic covalent bonds induce reversibility in polymer structures, which are characterized under appropriate conditions. These bonds are altered by external stimuli (light, heat, temperature, pressure, acidity and alkalinity), resulting in splitting and acting in a wide variety of scenarios [47,48]. Thermal reversibility has been widely utilized in the polymer field as an effective way to introduce healing and recyclability to the polymer field, thus effectively improving the mechanical properties of polymers [49]. Among several thermoresponsive bonds, the Diels–Alder (DA) reaction is one of the most widely studied. The reaction has a rich stereochemical presentation with both stereoselectivity, stereospecificity and regioselectivity through [4 + 2] addition reaction [50] between the electrically rich and electrically poor pro-diene, forming a new dynamic covalent bonding dienophile and pro-dienophile reagent reaction. The reaction is reversible, with lower reaction temperatures for forward ring formation and higher temperatures required for reverse ring opening reactions. Derivatives of furan (cyclodiene) and maleimide (cyclophilic diene) are ideal for DA reactions due to their electron-rich deficient nature and high reactivity [51,52].

The modification of polymers based on DA-bonded epoxides has been much studied and extensively described in the literature. The first report by Aubert [53] dating back to 2003 details the reaction of a di-epoxide compound formed by the DA reaction between two epoxy group-containing furans and bismaleimide (BMI), which yields epoxy-functionalized DA adducts; the adducts can be further reacted with aliphatic diamines. Since this early work, Chen et al. [54], Araya-Hermosilla et al. [55], and Pratama et al. [56] have investigated thermally reversible cross-linking systems for epoxy resins; however, practical considerations remain limited due to the use of solvents, encapsulation reagents (uncontrolled mixing), and long healing times. Alternative DA-based self-healing materials [57,58] consist mainly of elastomeric hydrogel formulations, and although they have high strain-to-damage ratios, they do not exhibit the high strength and stiffness requirements of high-performance thermoset epoxy materials.

While DA-based self-healing polymers show promise, their practical adoption in coatings is hindered by a fundamental compromise: elastomeric systems (e.g., hydrogels) lack mechanical rigidity, whereas rigid thermosets suffer from limited healing cycles due to irreversible side reactions. To address this issue, a semi-interpenetrating network (semi-IPN) strategy using furan-modified epoxy resin and bismaleimide (BMI) was proposed. This design leverages the high reactivity of furan-maleimide DA bonds for efficient healing; BMI’s thermal stability to resist retro-DA decomposition below 120 °C and its molecular architecture, enabling effective polymer network formation; epoxy’s inherent adhesion to porous substrates like wood. Crucially, the selected diglycidyl ether bisphenol A (DGEBA) epoxy provides cost-efficient raw material accessibility—reducing production costs while maintaining performance.

In this study, the synthesis of DA resins was carried out using mild ambient temperature (around 60 °C) in the case of tetrahydrofuran (THF) as a solvent to prepare novel self-healing, thermally reversible semi-interpenetrating polymer network (semi-IPN) materials consisting of furfuryl alcohol-maleimide modified epoxy resins, the scheme of which is illustrated in Figure 1.

Diglycidyl ether bisphenol A (DGEBA) shows an open chain structure internally, and at the ends of the molecular chain are closed epoxy groups, while furfuryl alcohol (FA) itself is a compound containing furan groups, and at high temperatures the closed epoxy groups of DGEBA react with furfuryl alcohol (FA) and open up the closed epoxy groups, thus introducing furan groups, to form furan-functionalized epoxy resins (FFERs) [59]. The DA reaction of the furanyl-functionalized epoxy resin with bismaleimide (BMI) was used to react the furanyl-functionalized epoxy resin with bismaleimide (BMI), resulting in the formation of a product containing an unsaturated six-membered ring, i.e., a DA polymer [60].

2. Materials and Methods

2.1. Materials

Diglycidyl ether bisphenol A (DGEBA), epoxy value 0.38–0.45 (Bidepharm Inc., Shanghai, China). Furfuryl alcohol (FA), 98% purity, triethylamine (TEA), 95.5% purity, bismaleimide (BMI), 98% purity, tetrahydrofuran (THF), 98%, and 4,4-diaminodiphenylmethane (DDM), 98% purity (all purchased from Shanghai McLean Biochemistry and Technology Co., Shanghai, China) (

Table 1. Test materials.

Test Material	Purity	Manufacturer
Diglycidyl ether bisphenol A	epoxy value 0.38–0.45	Shanghai Maclin Biochemical Technology Co., Ltd., Shanghai, China.
Furfuryl alcohol	98%	Shanghai Maclin Biochemical Technology Co., Ltd., Shanghai, China.
Triethylamine	95.5%	Shanghai Maclin Biochemical Technology Co., Ltd., Shanghai, China.
Bismaleimide diphenylmethane	AR	Shanghai Maclin Biochemical Technology Co., Ltd., Shanghai, China.
4,4-diaminodiphenylmethane	98%	Shanghai Maclin Biochemical Technology Co., Ltd., Shanghai, China.

).

2.2. Self-Healing Coating Preparation Method

A quantity of 2.38 g of DGEBA (containing 0.01 mol of epoxy groups) and 1.00 g of furfuryl alcohol (0.01 mol) were weighed separately. Both materials were placed into a 50 mL beaker. Then, 0.08 g of TEA catalyst was added to the beaker. The mixture was heated to 65 °C with continuous stirring. This reaction was maintained at 65 °C for 5 h. The resulting solution became viscous and transparent. The final product was a viscous liquid identified as furfuryl-functionalized epoxy resin (FFER) [61].

2.3. Preparation of DA Polymers and Semi-IPN Structured Polymers and Coating Production

The 1.83 g of bismaleimide (BMI, 0.01 mol) was weighed and dissolved in approximately 20 mL of THF within a beaker. The product from the previous step was added to this BMI solution. The mixture was then heated to 60 °C with continuous stirring and reacted for 5 min to ensure complete reaction, yielding the Diels–Alder (DA) polymer solution. This solution was brush-coated onto glass sheets and poplar boards. The coated substrates were placed in an electric constant-temperature drying oven and cured at 60 °C for 24 h to produce the self-healing coating (designated as Type-A self-healing resin).

A separate batch of the Diels–Alder (DA) polymer solution was prepared identically to the previous procedure. To this solution, the 2.38 g of DGEBA (containing 0.01 mol epoxy groups) was added. Subsequently, the 1.02 g of DDM (0.005 mol) was introduced and stirred at 60 °C until complete dissolution, forming a homogeneous solution after 5 min. This mixture was brush-coated onto glass sheets and poplar boards. The coated substrates were oven-dried at 60 °C for 24 h, yielding a semi-interpenetrating polymer network coating (designated as Type-B self-healing resin) [62].

A control epoxy resin was prepared as follows. First, 2.38 g of DGEBA (containing 0.01 mol epoxy groups) was added to a beaker. Then, 1.02 g of DDM (0.005 mol) was introduced. THF was added to fully dissolve both components. This solution was poured into silicone molds and cured at 60 °C for 24 h to obtain the control epoxy resin sample.

2.4. Characterization

Fourier Transform Infrared (FTIR) spectra of epoxy, FFER, Type-A, and Type-B resins were measured using a Bruker instrument model Equinox 55 spectrometer (Ettlingen, Germany), and FTIR spectra of polymers were measured using the attenuated total reflection (ATR) mode in the spectral range of 500–4000 cm⁻¹.

Gel Permeation Chromatography (GPC) was carried out on Agilent GPC Addon Rev. A.02.02 (Santa Clara, CA, USA) at room temperature using tetrahydrofuran as elution solvent to determine molecular weight and molecular weight distribution.

The Raman spectra were obtained using a Renishaw 2000 Raman Microscope (Gloucestershire, UK). The wavelength of the laser source was 785 nm, and the scattered light was dispersed with a grating of 1200 L/mm. The scattered laser light was collected by the Renishaw CCD camera. A 503 objective was chosen. The exposure time was set to 10 s with 1 accumulation. The laser power was 0.01 W. The glass slide was covered with aluminum foil to remove the disturbance from silicon oxide. The sample was placed on the foil-covered glass slide.

Thermogravimetric analysis (TGA) was carried out on a TGAPL, model TGA1500 (Mettler Toledo, Columbus, OH, USA) instrument (N₂ atmosphere) at temperatures ranging from 20 to 600 °C with a heating rate of 10 °C min⁻¹.

Scanning Electron Microscope (QUANTA-200, Thermo Fisher Scientific Co., Ltd., Waltham, MA, USA) (SEM): The treated sample was fixed on the sample plate with double-sided tape for gold spraying treatment. After gold spraying, the sample was placed on the observation platform inside the scanning microscope for vacuum operation, and the air pressure was adjusted to the appropriate level for observation.

Color difference: The color difference meter was used to obtain the L, a, and b values of the coating film. L value represents the sample’s brightness value, a value represents the sample’s red-green value, and b value represents the sample’s yellow-blue value. The test data of the coating film in the control group were L₁, a₁, b₁, and the test data of the coating film with microcapsules were L₂, a₂, b₂. The color difference ΔE was calculated according to Formula (1), where ΔL = L₂ − L₁, Δa = a₂ − a₁, and Δb = b₂ − b₁, and the average value of each group of data is measured three times.

$$\mathsf{\Delta }E={[{\left(\mathsf{\Delta }L\right)}^2+{\left(\mathsf{\Delta }a\right)}^2+{\left(\mathsf{\Delta }b\right)}^2]}^{\frac{1}{2}}$$

(1)

Gloss: Gloss testing is widely used in the testing of materials and products in many different fields, which is particularly important for the evaluation of the performance and quality of coating films.

Transmittance and reflectance: The transmittance and reflectance of the coating film in the visible wavelength range were tested using an ultraviolet spectrophotometer (U-3900, Hitachi Instrument Co., Ltd., Suzhou, China).

Self-healing performance test: The self-healing performance of the coating film was tested using the scratch test. The surface of the coating was scratched with a razor blade, and the width of the scratch was observed and recorded using an optical microscope, at which time the width of the scratch was recorded as W₁. The scratch at the location was observed again after 48 h, and the width of the scratch was recorded as W₂. The self-healing rate of the coating film was calculated from W₂ and W₁. The formula for the self-healing rate H is shown in Equation (2).

$$H={\displaystyle \frac{W_1-W_2}{W_1}\times 100\%}$$

(2)

3. Results and Discussion

3.1. Chemical Analysis of Resins

The GPC test is used to determine the molecular weight information in a synthetic resin and is used to evaluate the success of the reaction. Figure 2 shows the GPC results and

Table 2. Molecular weight test results.

Number Average Molecular Weight Mn	Weight Average Molecular Weight Mw	Peak Molecular Weight MP	Z-Mean Molecular Weight Mz	Z + 1 Average Molecular Weight Mz + 1
409	1717	155	5564	9989

summarizes the molecular weight data. The low-molecular-weight peaks indicate the presence of a large number of small molecules, possibly including unreacted monomer or solvent molecules, and the high PDI values indicate the presence of a large number of both small- and large-molecular-weight molecules. A high Z-mean molecular weight also indicates the presence of large-molecular-weight molecules. All of these results indicate that there is an incomplete reaction in the synthesis process. For self-repairing polymers, such results imply a lower degree of polymerization, which can affect the mechanical strength of the polymer, and the simultaneous presence of large- and small-molecular-weight molecules may affect the homogeneity of the polymer network.

FTIR testing utilizes the absorption of chemical bonds at specific infrared wavelengths and is used to determine molecular information and verify experimental results. Figure 3 shows the FTIR results. Characteristic furan absorption occurs at 740 cm⁻¹ (monosubstituted furan), 1147 cm⁻¹ and 1080 cm⁻¹ (furan ring breathing), verifying the bonding of the furan to the epoxy backbone. Meanwhile, the epoxy feature at 862 cm⁻¹ (symmetric epoxy stretching) disappears, while a new broad hydroxyl band appears at 3450 cm⁻¹ [63,64,65].

FTIR analysis confirmed successful Diels–Alder polymer synthesis through characteristic peak changes. The spectrum of Type-A self-healing resins shows diagnostic absorptions at 1705 and 1760 cm⁻¹, corresponding to the succinimide ring formed via the Diels–Alder reaction between maleimide and furfuryl groups. Concurrently, key reactant peaks disappeared: maleimide deformation at 682 cm⁻¹, maleimide ring C-H stretch at 3102 cm⁻¹, furan ring signals at 740 cm⁻¹ and 1147 cm⁻¹. These reciprocal spectral changes verify complete consumption of reactants and formation of the DA adduct.

The FTIR spectrum of the semi-interpenetrating network (Type-B self-healing resin) demonstrates characteristic features from both DDM-cured epoxy (DGEBA/DDM system) and Diels–Alder polymer (FFER/BMI system). Key evidence for reaction completion includes (1) disappearance of the DGEBA epoxy signature at 862 cm⁻¹ (symmetric epoxide ring stretch) and the FFER furan peak at 740 cm⁻¹ (out-of-plane C-H bend), and (2) emergence of new Diels–Alder adduct absorptions at 1705 and 1760 cm⁻¹ (succinimide ring vibrations). These reciprocal spectral changes confirm successful semi-IPN synthesis [66,67].

Raman spectroscopic detection utilizes the elastic and inelastic reflections of incident light on a substance, and by detecting the inelastic reflected light, it can be used to detect the substance and verify the experimental results. Raman spectroscopy in Figure 4 confirms successful Diels–Alder bond formation. A new peak at 642 cm⁻¹ corresponds to the six-membered DA adduct ring structure. The 938 cm⁻¹ peak arises from symmetric C-O-C stretching in the ether linkage formed during the DA reaction between bismaleimide and furan groups. Crucially, the absence of signals between 1500–1520 cm⁻¹ and 1570–1590 cm⁻¹ confirms complete furan group consumption, while missing bismaleimide C=C stretches at 1590–1610 cm⁻¹ further verify DA reaction completion. However, residual epoxy groups detected at 840 cm⁻¹ indicate incomplete initial grafting of the epoxy resin. Additionally, the 1750 cm⁻¹ peak signifies carboxylic acid formation from degradation of the furfuryl-functionalized epoxy resin, revealing unintended side reactions.

3.2. Thermodynamic Analysis of Polymers

TGA testing is the process of examining the change in material quality by exposing the material to elevated temperatures, which can be used to determine the thermal stability and decomposition behavior of the material at different temperatures. Figure 5 shows the results of the TGA test for both resins. The results show that both resins decompose between 300 and 400 °C due to ether chain cleavage and aromatic ring decomposition. The initial decomposition and 10% weight loss temperatures are consistently between 300 and 380 °C. Type-B shows a slight decomposition behavior below 300 °C, probably due to the decomposition of a small portion of the solvent or unreacted monomers. During heating, the inverse Diels–Alder reaction releases free maleimide and furan fractions from the DA adduct and the semi-IPN network. As a result, the thermal stability of Type-B resins is superior to that of Type-A resins.

3.3. Coating Optical Property Analysis

Color difference (ΔE) analysis was conducted at multiple points on coated substrates, with lower ΔE values indicating superior coating homogeneity. Gloss measurement and visible-light transmittance/reflectance represent critical optical properties; higher gloss units (GU) correlate with enhanced surface quality, while increased transmittance in the visible spectrum (380–780 nm) enables clearer visualization of substrate texture features.

Table 3. Color difference analysis of glass plate coating and poplar boards.

Coating	Different Position	L	a	b	ΔE
Type-A resin surface coating on glass plate	Point 1	40.6	44.9	28.4	-
	Point 2	39.9	45.3	27.9	-
	chromatic aberration	-	-	-	0.8
Type-B resin surface coating on glass plate	Point 1	63.3	14.4	38.9	-
	Point 2	64.2	16.6	37.5	-
	chromatic aberration	-	-	-	2.7
Type-A resin surface coating on poplar boards	Point 1	46.1	41.2	22.1
	Point 2	46.6	41.4	24.1
	chromatic aberration				2.0
Type-B resin surface coating on poplar boards	Point 1	52.1	30.1	43.7
	Point 2	53.2	29.2	44.5
	chromatic aberration				1.6

data reveal minimal color variation on glass substrates, with all ΔL, Δa, Δb, and ΔE values below 1.0. According to color difference standards, ΔE < 1.0 indicates visually imperceptible variation. The glass coating’s measured ΔE of 0.8 confirms exceptional color uniformity, high surface smoothness, and superior film consistency. Conversely, the poplar board coating shows ΔE = 2.0. Within the 1.0–2.0 ΔE range, color differences become marginally detectable only by observers with high color sensitivity. Thus, while poplar coatings exhibit measurable color variation, it remains within functionally acceptable limits with maintained surface uniformity and flatness.

Table 3 data indicate greater color variation on glass substrates versus poplar counterparts. Glass coatings show ΔE = 2.7, within the 2.0–3.0 range, where color differences become marginally perceptible. This confirms a generally uniform color distribution but inferior performance relative to the reference self-healing epoxy. Conversely, poplar coatings exhibit superior uniformity with ΔE = 1.6, below the 2.0 threshold for noticeable variation. This places poplar substrates in the 1.0–2.0 ΔE range where color differences are only faintly detectable by trained observers. Both substrates maintain good film smoothness, but poplar demonstrates higher overall coating quality through lower measurable color variation.

The data in

Table 4. Glossiness of self-healing resin coating film.

Materials	20° Gloss (%)	60° Gloss (%)	85° Gloss (%)
Poplar board with Type-A resin	15.2	52.8	48.8
Glass pane with Type-A resin	97.9	115.3	91.6
Poplar board with Type-B resin	96.8	106.7	97.4
Glass plate with Type-B resin	152.2	140.4	99.4

shows that the gloss of the coatings on the wood substrate was lower than that on the glass substrate, but all samples maintained a high gloss. This indicates that the surface morphology of the coatings is excellent with low roughness. Maximum gloss for all samples occurred at the standard 60° measurement angle, and Type-B resins showed excellent gloss retention on both substrates. These findings conclusively demonstrate that Type-B resins with semi-IPN structures can produce coatings with excellent gloss characteristics.

Figure 6 compares the light transmission and reflectance of two self-healing coatings on glass substrates. The Type-B resin provides better light transmission compared to the Type-A resin. This is due to the highly ordered molecular arrangement in the epoxy network contained in the Type-B resin, which minimizes light scattering. Unlike the light transmission, the reflectance values of the two materials are comparable (Type-A: 38.51%; Type-B: 40.20%). It is noteworthy that the visible spectral reflectance curve of Type-B is more stable, despite the difference of 1.69% in absolute value, which indicates a better surface homogeneity.

3.4. Self-Healing Properties of Polymers

Poplar boards and glass boards were coated with the prepared Type-A self-healing resin. After the resin had fully cured, scratches were made on the coated surfaces. These scratches were about 15 mm long. The scratches were then heated at 120 °C for 30 min to allow the scratches to heal. Figure 7 shows how the scratches looked before healing. The images show that scratches became much lighter after heat treatment. Some scratches even disappeared completely. This result shows that the Type-A self-healing resin has excellent self-healing ability.

On glass, scratches healed almost completely after heat treatment. Only a few small air bubbles remained inside the coating. This shows high healing efficiency. The glass surface is very smooth and does not absorb water. These properties give the coating molecules an ideal surface to move and rearrange on. In contrast, the healing effect was much worse on wood. During the coating process, the wood absorbed water. This caused the wood to soak up too much resin solution. The result was an uneven coating thickness. Also, during heat treatment, the moisture inside the wood evaporated. This created many bubbles. These bubbles broke the coating film and made it hard to see or evaluate the healing effect. Furthermore, the wood’s rough surface fibers made surface observation even more difficult. The main limitation of this study is how the wood’s porous structure and ability to absorb water affected the healing process. Future studies could improve the results. Possible improvements include sealing the wood first, optimizing how the coating is applied, and controlling the humidity of the surrounding air.

The self-healing epoxy coating healed surfaces differently depending on the underlying material. As

Table 5. Self-healing rate of different scratches (Type-A resin).

Scratch Number	Scratch Width (μm)	Scratch Width After 30 min (μm)	Self-Healing Rate (%)
Slide 1 on glass plate	89.96	14.00	84.43
Slide 2 on glass plate	142.48	94.52	33.66
Slide 3 on glass plate	133.05	32.91	75.26
Slide 1 on poplar boards	123.70	49.83	59.71

shows, the coating’s average healing rate was only 59.71% on wood. This was much lower than its 84.43% healing rate on glass. Three main reasons explain this difference. First, the glass surface is smooth and does not absorb water. This smoothness helps coating molecules move and reconnect easily. Second, the razor blade’s condition affected healing results. The first and third scratches showed healing rates over 80% because the blade made clean, regular cuts. However, the second scratch had a lower healing rate because the worn blade edge created rough scratch edges. Third, wood’s natural fibers and water-absorbing properties further reduced healing effectiveness. These results prove that both material properties and experimental conditions significantly impact the coating’s healing performance.

Poplar boards and glass boards were coated with the prepared Type-B self-healing resin. And it was treated as in the case of Type-A resin. Then these scratches healed by heating them at 120 °C for 30 min. Figure 8 shows how the scratches looked before healing. The images reveal that scratches became lighter after heat treatment. This change confirms that the Type-B resin has self-healing ability. However, this healing ability is weaker than that of the Type-A resin.

The results show that Type-B self-healing resin heals scratches effectively on different surfaces. While its healing efficiency on wood is lower than on glass, it still performs much better than Type-A resin on wood. This advantage comes from Type-B resin’s semi-IPN structure. The added epoxy resin works in two key ways. First, its rigid network makes the material much stronger while still allowing good self-healing, even though molecules move slightly slower. Second, the epoxy forms a tight seal on wood surfaces. This seal blocks moisture from entering the wood and strengthens the bond between the coating and wood fibers. Together, these effects reduce how wood’s water absorption interferes with healing. They also create better stress transfer, giving a good balance between healing ability and material strength.

Table 6. Self-healing rate of different scratches (Type-B resin).

Scratch Number	Scratch Width (μm)	Scratch Width After 30 min (μm)	Self-Healing Rate (%)
Slide 1	131.81	40.64	69.17
Slide 2	164.56	55.24	66.43
Slide 3	115.55	45.97	60.22
Poplar plank	52.97	23.62	55.41

shows that Type-B self-healing resin maintains stable healing performance across different scratch types. This stability comes from its semi-IPN polymer network. The network works through two combined actions. First, the rigid epoxy framework gives the material excellent strength and high-temperature stability. This rigid structure controls how cracks spread during damage, creating cleaner breaks that are easier to heal. Second, the Diels–Alder (DA) reversible bonds in the network enable crack healing through thermal activation. However, the epoxy framework slightly restricts the molecular movement needed for DA bond healing. This restriction makes Type-B resin healing itself slightly slower than Type-A resin within 30 min. Overall, the semi-IPN design gives Type-B resin a major performance advantage.

SEM allows direct observation of coating surface morphology and healed crack closure at the microscopic level. With cross-section preparation, it further allows observation of the interface between the coating and the substrate. Figure 9 compares the surface appearance of glass substrates coated with Type-A and Type-B self-healing resins before and after healing. For Type-A resin, heating breaks its abundant Diels–Alder (DA) bonds. These bonds then reconnect during cooling to fully heal cracks. SEM images show that the healed areas look identical to the original coating. This confirms that Type-A resin has excellent self-healing ability. In contrast, Type-B resin contains fewer DA bonds due to its semi-IPN network design. During healing, DA bonds break, but molecular movement is restricted by the rigid epoxy framework. This limited movement prevents complete crack healing. There is about 30–40% microcracks remaining in the healed areas.

Figure 10 shows SEM images comparing how Type-A and Type-B self-healing resins bond to wood surfaces. The Type-A resin has strong self-healing ability from its Diels–Alder (DA) bonds. This limits its bonding to wood. During curing, moisture escaping from the wood created internal stress. This stress caused localized cracks in the Type-A coating. Glass surfaces showed no cracking since they do not absorb water. In contrast, Type-B resin bonds much better to wood. Its rigid epoxy network improves the coating’s strength and grips wood fibers tightly. This makes the wood-coating bond stronger. SEM images reveal a key reason: Type-B resin forms a continuous, dense layer between the coating and wood. This layer effectively blocks moisture movement that causes cracking and interface failure.

Unlike the hexagonal boron nitride (hBN)/epoxy of Cao et al. [68], the application of self-healing resins as coatings to the special material of wood was broadened, which is a completely new field. Wood, due to its porous, hygroscopic characteristics and its patterned nature, requires even more protection from coatings as a material. In addition, Type-B resin was designed by utilizing a semi-IPN structure, which integrates DA resin into the epoxy resin network to enhance the mechanical properties of the material and better protect the substrate. And the results show that the Type-B resin is superior to the Type-A resin in color difference, gloss, and thermal stability after sacrificing a certain amount of self-healing ability. This work broadens the application scope of self-healing materials and provides a new idea for the research and development of self-healing materials, which has broad prospects.

4. Conclusions

The aim of this study was to create a new type of self-healing polymer. Two different self-healing resins, Type-A resin and Type-B resin, were synthesized. The successful formation of DA bonds in the self-healing resins was confirmed using FTIR and Raman spectroscopy. However, GPC analysis revealed the shortcomings of the experiments; the excess of low-molecular-weight molecules and high PDI values indicated incomplete reaction and low yield, leaving room for improvement. The TGA results illustrated that the semi-IPN structure of the Type-B resin gave it a higher heat resistance compared to the Type-A resin. Both resin coatings performed well in terms of self-healing efficiency. On glass, Type A resin achieves about 80% scratch healing after heating. On wood, however, due to the material properties of wood, the healing rate drops to about 60%. Type-B resin heals scratches on glass at about 65%. On wood, the healing rate was about 55%. SEM observations confirmed this pattern; in general, the Type-A resins healed better, but the Type-B resins bonded more tightly to the wood and were more suitable for use as a coating. Thus, while Type-A resins are more capable of healing, Type-B resins are physically stronger and better adapted to the environment, especially on challenging surfaces such as wood. Both resin coatings have good optical properties. Type-A resin has a small color variation: ΔE = 0.8 on glass and ΔE = 2.0 on wood. Type-B resin also has a small color variation: ΔE = 2.7 on glass and ΔE = 1.6 on wood. These small color differences indicate that both resins form a homogeneous coating with a good surface quality. Gloss measurements differed somewhat between the two resins, with Type-A resins producing higher gloss on glass (97.9 GU at 20°, 115.3 GU at 60°, and 91.6 GU at 85°) than on wood (15.2 GU, 52.8 GU, and 48.8 GU, respectively, for the corresponding angles). On the contrary, the semi-IPN structure of Type-B resins produced higher gloss on wood (152.2 GU, 140.4 GU, 99.4 GU) than on glass (96.8 GU, 106.7 GU, 97.4 GU). In conclusion, although the self-healing ability of the Type-B resin is weaker than that of the Type-A resin, it has better heat resistance, stronger bonding with the wood material, better gloss properties, and good self-healing ability.