Preparation of Tung Oil-Modified Raw Lacquer Films and Application for Mechanical Carving Technique

Abstract

Raw lacquer, known for its superior performance as a natural liquid coating, boasts excellent physical and mechanical properties as well as durability, making it widely used in manufacturing. However, the high hardness of the lacquer film upon complete curing poses challenges for carving and mechanical engraving. Therefore, it is necessary to study the curing process of lacquer films to obtain films suitable for carving or mechanical engraving. This study involves the preparation of raw lacquer with varying amounts of tung oil added, followed by the measurement of film drying time, surface roughness, glossiness, hardness, and adhesion on substrates to determine the optimal drying conditions. Additionally, SEM analysis of the carved surfaces and FT-IR analysis were used to investigate the impact of tung oil addition on lacquer carving performance and its variation. The results indicate that tung oil, to a certain extent, contributes to a smoother lacquer film but adversely affects film hardness and adhesion to Prunus serotina. However, with an increase in the amount of refined tung oil to 15%, the film exhibits improved glossiness, smoother carving tool marks, and reduced debris, thereby validating the feasibility of mechanical carving of tung oil-modified raw lacquer to some extent.

1. Introduction

The depletion of fossil fuels, their increasing costs, and associated environmental impacts have accelerated the pursuit of biomass materials and renewable resources. The use of green, environmentally friendly and renewable materials is the trend [1,2]. Lacquer, as a natural organic polymer coating, not only offers environmental benefits but is also a renewable resource with virtually unlimited availability. China, being the source of over 70% of global lacquer production annually, faces the challenge of optimizing the utilization of this resource [3]. Lacquer, also known as oriental lacquer or raw lacquer, is a high-quality natural coating derived from the sap of the lacquer tree (Toxicodendron vernicifluum). Renowned for its exceptional durability, lacquer is often referred to as the “king of coatings” due to its superior longevity compared to modern synthetic alternatives [4,5]. With the deepening of modern research on raw lacquer, its applications have increasingly expanded. Raw lacquer has become an important material in modern industries, agriculture, national defense, and technology. For instance, its anti-fouling and antimicrobial properties make it suitable for wide applications in marine anti-fouling, medical fields, and certain industrial coatings [6,7,8]. Ordinary coatings require the addition of antimicrobial agents to achieve antimicrobial properties [9,10], while raw lacquer itself has antimicrobial properties. Water-based paints widely used in daily life are prone to cracking [11,12,13], while raw lacquer is even less prone to cracking. Thus, as an excellent material for decoration and protection, raw lacquer can be broadly applied in fields such as chemicals, dyeing, crafts, and high-level furniture [14,15].

Chemically, lacquer is a white, viscous liquid composed of an oil-in-water emulsion. Its primary constituents include urushiol (40%–80%), urushiase (less than 1.0%), urushi polysaccharides (5%–7%), glycoproteins (2%–5%), and water (15%–40%) [16]. Urushiol, the principal film-forming agent, contains phenolic structures and exhibits chemical reactivity that allows for modification. Urushiase, a glycoprotein, facilitates the oxidative polymerization of urushiol, which is essential for the lacquer film’s room-temperature drying process. Urushi polysaccharides stabilize the emulsion and enhance the lacquer film’s properties, while water plays a crucial role in both the enzymatic catalysis and the ionization of urushiol. Raw lacquer is rich in benzene ring structures within its lacquer phenol. During film formation, the unsaturated side chains of lacquer phenol undergo continuous oxidation and cross-linking, leading to the formation of a stable interpenetrating network polymer. Consequently, the natural raw lacquer exhibits reduced flexibility and impact resistance. Additionally, the limited production yield and relatively high cost of raw lacquer, along with certain physical properties such as stringent drying conditions, restrict its current production efficiency and broader application [17].

Tung oil, or China wood oil, derived from the seeds of the tung tree, is characterized by its fast-drying properties, strong adhesion, glossiness, resistance to water and heat, electrical insulation, acid resistance, and corrosion resistance. Historically utilized in China, the refined tung oil is often blended with varnishes or coatings to enhance film performance [18]. This refined tung oil, which is darker and more viscous than raw tung oil, improves lacquer film properties due to its rapid oxidation and polymerization characteristics [19,20]. The practice of mixing refined tung oil with lacquer represents one of the earliest modification techniques aimed at reducing lacquer costs while enhancing film gloss and adhesion [21,22]. Carving lacquerware, a traditional craft in China, has long been primarily done by manual carving. Due to the drying characteristics of the lacquer film, carving must be performed before the lacquer layer is completely dry to avoid difficulties arising from excessive hardness once the lacquer has fully cured. This requirement significantly impacts both the production efficiency and the broader dissemination of this craft [18]. Therefore, the integration of lacquer carving with custom furniture and the application of advanced mechanical carving techniques alongside traditional methods is vital [23,24].

In this study, the tung oil-modified raw lacquer films were prepared by mixing refined tung oil with raw lacquer in varying proportions and drying them under conditions of 25 °C and 80% humidity. The physical properties of the modified lacquer films, including drying time, glossiness, and hardness, were tested. Based on the test data, a sculptural specimen with a coating thickness of 15 mm was created for simulation tests of mechanical carving. Furthermore, both the carved cross-section and the film composition were subjected to mesoscopic and microscopic analysis, providing a preliminary validation of the feasibility of mechanical carving using tung oil-modified raw lacquer.

2. Test Materials and Methods

2.1. Materials and Tools

(1)

Materials

The raw lacquer used in this experiment was Shanxi Ankang raw lacquer (Yangzhou Lacquerware Factory Co., Ltd., Yangzhou, China), while the refined tung oil was of National Standard Grade I (Haitang Hongyang Tung Oil Cooperative Co., Ltd., Xinyang, China). The purity of the anhydrous sodium phosphate (Xilong Scientific Co., Ltd., Guangzhou, China) used to prepare the saturated solution was of analytical reagent (AR) grade. The Prunus serotina blocks (100 mm × 100 mm × 10 mm) were pre-sanded and cleaned using #800 sandpaper before coating.

(2)

Tools

Table 1. List of experimental tools.

Tools	Manufacturer
Desiccator	Nanjing Luanyu Chemical Glass Instrument Co., Ltd., Nanjing, China
Heating Magnetic Stirrer	Shangpu Instrument Equipment Co., Ltd., Changzhou, China
Filter Paper	Shanghai Guobei Filter Paper Factory Co., Ltd., Shanghai, China
Precision Electronic Balance	Shanghai Precision Scientific Instrument Co., Ltd., Shanghai, China
Roughness Tester	Shanghai Taijing Optical Instrument Co., Ltd., Shanghai, China
Hygrometer	Guangdong Meidashi Instrument and Meter Co., Ltd., Guangzhou, China
Thermometer	Guangdong Meidashi Instrument and Meter Co., Ltd., Guangzhou, China
Pencil Hardness Tester	Shanghai Pushen Chemical Machinery Co., Ltd., Shanghai, China
Gloss Meter	Shenzhen Sanen Technology Co., Ltd., Shenzhen, China
Coating Adhesion Tester	Shenzhen Sanen Technology Co., Ltd., Shenzhen, China
Ultrasonic Coating Thickness Gauge	DeFelsko Coporation Co., Ltd., Ogdensburg, NY, USA
3D CNC Machine	Dongguan Taichuan CNC Technology Co., Ltd., Dongguan, China

shows the list of experimental tools, the sample preparation tools and certain testing instruments are included.

2.2. Sample Preparation

(1)

Preparation method of desiccator

The 100 g of disodium hydrogen phosphate (Na₂HPO₄) was weighed and dissolved in 400 mL of ultrapure water in a beaker. Ultrapure water was gradually added to the disodium hydrogen phosphate while the solution was heated and stirred on a magnetic stirrer set to 600 rpm at 60 °C. Heating and stirring were continued until the solution became completely clear and no additional Na₂HPO₄ could dissolve. The saturated solution was allowed to cool to room temperature (25 °C) before being transferred to a desiccator. The desiccator was sealed with petroleum jelly, and a hygrometer was inserted to monitor the humidity. Once the humidity levels stabilized at 80 ± 3% respectively, the hygrometers were removed, and the desiccators were stored for future use.

(2)

Preparation method of mixing raw lacquer and preparing coating film

The raw lacquer was filtered through a 65 µm filter paper to remove natural impurities and solid particles. The 10 g of raw lacquer was weighed and mixed with 0.5 g, 1.5 g, and 3.0 g of tung oil at room temperature (25 °C) to prepare lacquer solutions with tung oil concentrations of 5%, 15%, and 30%, respectively. Twelve glass slides were prepared and labeled according to the tung oil content as RL (raw lacquer without tung oil), 5-T,15-T and 30-T, with three samples assigned to each group. The initial mass of each slide was weighed and recorded.

A measured amount of the mixed lacquer solution was applied to one end of each slide using a pipette. The lacquer was spread evenly using the 100 µm side of a four-sided film applicator, ensuring that each slide received a single, uniform coating. After coating, the slides were reweighed to confirm that the film mass and thickness were consistent across samples. The slides were then placed in desiccators under different humidity conditions and allowed to dry. The mass of each slide was measured every 0.5 h, and the drying changes in the lacquer film were observed. The drying times and physical properties were recorded.

(3)

Carving method for lacquered film

Three Prunus serotina blocks (100 mm × 100 mm × 10 mm) were cleaned and positioned on a workbench. Lacquer with tung oil contents of 0%, 5%, and 15% was applied to the wood blocks using a brush in a clockwise direction. The coated wood blocks were then allowed to dry in a desiccator. After the surfaces were dry, the coating effects were inspected, necessary adjustments were made, and the process was repeated until the film thickness reached 15 mm. Mechanical processing was conducted using a 3D CNC machine. The carving performance and microscopic properties of the lacquered film were analyzed.

2.3. Lacquered Film Performance Testing

(1)

Drying time measurement

The surface and actual drying times of the coating film were measured according to GB/T1728-2020 [25]. The surface drying time is determined when the film feels slightly tacky to the touch but no paint adheres to the finger. The actual drying time is determined when pressing with a degreased cotton ball for 30 s leaves 1–2 fibers, which can be lightly brushed off.

(2)

Test for glossiness of coating

The sample was treated according to the requirements of GB/T 4893.6-2013 [26]. The glossiness values of the coating at three incidence angles of 20°, 60°, and 85° were tested and recorded using a glossmeter (Shenzhen Linshang Technology Co., Ltd., Shenzhen, China), with the unit being GU [27]. Each sample was measured in triplicate at each incident angle, and the average value was recorded for analysis.

(3)

Roughness testing of coating

According to the national standard GB/T1031-2009 [28], a touch-probe precision roughness tester was used to measure the roughness of coating films under different conditions, with R_a (Arithmetic Average Deviation of the Profile) and R_q (Root Mean Square Deviation from the Profile Mean Line) as the parameters reflecting roughness changes. R_a values, indicating the overall roughness of the coating film, were recorded and analyzed for samples with varying tung oil concentrations. The macro knob was rotated to fine-tune the probe position until the red display point was at the zero-scale line. A test button was clicked, and data were recorded. The unit of roughness value is µm. The average value for each set is computed and recorded for subsequent analysis.

(4)

Hardness and adhesion testing of coatings

According to GB/T 6739-2022 [29], a pencil with a hardness of 9B-9H was used, which was provided by a QHQ-A portable Pencil Hardness Tester (Shanghai Pushen Chemical Machinery Co., Ltd., shanghai, China). The pencil was inserted diagonally at a 45° angle into the pencil hardness tester with a load of 750 g for hardness testing. Each sample was tested once, with three samples per group. The coating film hardness was determined as the highest pencil hardness that did not result in observable damage to the film.

According to GB/T 4893.4-2013 [30], the adhesion of the coating was tested using a coating adhesion tester (Quzhou Aipu Measuring Instrument Co., Ltd., Quzhou, China). The coating was cross-cut with the blade at a vertical angle of 90 degrees. A 3M adhesive tape was applied on the grid surface and quickly and smoothly peeled off at an angle close to 60°. Each set of samples was tested three times, and the optimal value was recorded. The adhesion level decreased from level 0 to level 5.

(5)

Scanning electron microscopy (SEM)

The morphology of the coated samples was observed and characterized using an FEI Quanta 200 scanning electron microscope (FEI Company, Inc., Beaverton, OR, USA) at an acceleration voltage of 5.0 kV. Additionally, an elemental mapping (EDS-Mapping) was performed at 10 kV to obtain the spatial distribution and semi-quantitative analysis of elements [31,32].

(6)

Fourier-transform infrared spectroscopy (FTIR)

Structural characterization of the modified and unmodified samples was conducted using a Bruker VERTEX 80V infrared spectrometer (Brucker AG, Karlsruhe, Germany). The testing parameters were set as follows: a wavenumber range of 400–4000 cm⁻¹, 32 scans, and a resolution of 4 cm⁻¹ [33,34].

3. Results and Discussion

3.1. Effect of Tung Oil Addition on Coating Film Drying Time

Based on the experimental results, it was observed that the surface drying time was shortest at 2.5 h when the tung oil content was 5%. When the tung oil content was 15%, the surface drying time increased to 5 h. At a tung oil content of 30%, the surface drying time was the longest, taking 8 h. This indicates that the drying time of the coating film increases with the amount of tung oil added [35].

Initially, the lacquer enzyme oxidizes lacquer phenol to reactive lacquer phenol radical intermediates under O₂ conditions. These phenol radical intermediates then undergo cross-linking reactions with phenol monomers to form phenol dimers. Subsequently, the phenol dimers further undergo oxidative cross-linking reactions catalyzed by the lacquer enzyme, resulting in the formation of long-chain or network polymer structures [16]. Tung oil is a thermally polymerizing plant oil that can oxidize and cure at room temperature. During the oxidation process, the −H in the unsaturated fatty acid chains reacts with oxygen to form hydroperoxides. These hydroperoxides readily decompose into stable free radicals, which then undergo polymerization, leading to curing. This curing process takes longer than that of raw lacquer. Therefore, the experimental results clearly show that as the amount of tung oil increases, the drying time of the coating film also extends [36]. Drying time directly affects the efficiency of lacquer application, which in turn impacts the production efficiency of carved lacquer. Therefore, the drying time of the lacquer film should not be excessively prolonged.

3.2. Effect of Tung Oil Addition on Coating Film Roughness

The roughness of a coating film is characterized by its microscopic geometric structures, including small spacing and minor peaks and valleys on the surface. As shown in Figure 1, both R_a and R_q values decreased with increasing tung oil content. This reduction is due to the lower inherent roughness of tung oil compared to raw lacquer. As the tung oil content increases, the coating film’s roughness decreases accordingly. This effect may result from the distinct curing processes of raw lacquer and tung oil, which lead to molecular separation during curing, causing the coating film surface to become smoother and reducing its gloss [37].

3.3. Glossiness of Coating Films with Different Tung Oil Concentrations

According to the national standard GB/T 4893.6-2013 [26], the gloss of coating films was determined using a glossmeter, and the resulting gloss data were recorded (

Table 2. Glossiness of coating films with different tung oil concentrations.

Sample Name	20° (GU)	60° (GU)	85° (GU)
RL-1	0.9	10.4	33.8
RL-2	1.1	11.3	35.3
RL-3	0.7	9.5	32.3
5-T-1	0.3	3.3	11.3
5-T-2	0.2	2.8	4.7
5-T-3	0.7	7.5	16.7
15-T-1	2.7	23.1	64.9
15-T-2	2.8	23	68.4
15-T-3	3.5	27.5	78.8
30-T-1	0.6	5.1	13.2
30-T-2	0.6	5.4	14.0
30-T-2	0.6	5.1	13.6

). The gloss measurement was conducted at three angles: 20°, 60°, and 85°. When measuring low-gloss materials with a 20° instrument, the detected light flux was very weak, resulting in very small measurements that make it difficult to distinguish differences in gloss between materials. Conversely, when measuring high-gloss materials with an 85° angle, the data also showed a limited ability to differentiate gloss levels among materials. Consequently, a 60° angle was typically chosen for the gloss measurement.

$$SSB={\sum }_{i=1}^{n_i}\hspace{1em}{n_i\left(x_{ij}-{\overline{x}}_i\right)}^2$$

(1)

$$SSW={\sum }_{i=1}^K\hspace{1em}{\sum }_{j=1}^{n_i}\hspace{1em}{\left(x_{ij}-{\bar{x}}_i\right)}^2$$

(2)

$$MSB={\displaystyle \frac{SSB}{k-1}}$$

(3)

$$MSW={\displaystyle \frac{SSW}{N-k}}$$

(4)

$$F={\displaystyle \frac{MSB}{MSW}}$$

(5)

According to the experimental results, when the tung oil was absent from raw lacquer, the gloss of the dried coating film at 80% humidity was 10.4. The gloss meter readings at an incident angle of 60° were subjected to variance analysis. Using Formulas (1)–(4), the between-group variance (SSB), within-group variance (SSW), between-group mean square (MSB), and within-group mean square (MSW) were calculated. The resulting F-value was approximately 71.6, with degrees of freedom of 3 and 8 and a p-value less than 0.001. This indicates that different amounts of tung oil significantly affected the gloss of the coating film. When tung oil content is 5% or 30%, the gloss is lower than that of ordinary raw lacquer. However, with a tung oil content of 15%, the coating film exhibits optimal gloss. The production of lacquerware often demands a high level of gloss, which suggests that the addition of tung oil to raw lacquer can significantly influence the gloss of the coating film. This effect is likely due to tung oil being a drying oil that, when mixed with raw lacquer, undergoes cross-linking and curing with oxygen during the drying process.

3.4. Effect of Tung Oil Addition on Coating Film Hardness

The hardness of coating film is a critical parameter in assessing its physical properties. During the carving of lacquerware, to avoid difficulties in processing caused by excessive hardness of the lacquer layer once it is fully dried, operators typically work while the lacquer is partially cured but still retains some elasticity. As per the national standard GB/T 6739-2022 [29], hardness measurements are recorded using a pencil hardness tester (refer to

Table 3. Hardness of the coating films with varying tung oil additions.

Sample Name	Hardness
RL-1	3H
RL-2	3H
RL-3	3H
5-T-1	2H
5-T-2	2H
5-T-3	2H
15-T-1	H
15-T-2	H
15-T-3	H
30-T-1	B
30-T-2	B
30-T-3	B

). Observations indicate that, in the absence of tung oil and at a humidity of 80%, the coating film achieved an optimal hardness of 3H. However, the inclusion of tung oil results in a notable decrease in hardness: from 2H with a 5% tung oil concentration to H with a 15% concentration and further to B with a 30% concentration. This trend demonstrates that both the addition of tung oil and its concentration significantly influence the hardness of the coating film, with increasing tung oil content correlating with reduced hardness. In particular, a higher amount of tung oil addition may result in a coating film hardness that is lower than H after drying. This could have implications for the processing and subsequent use of the lacquerware.

3.5. Effect of Tung Oil Addition on Coating Film Adhesion

The adhesion, defined as the capacity of the coating film to bond with the substrate surface, is a crucial factor in evaluating coating film durability (refer to

Table 4. Adhesion grade of coating films with varying tung oil additions.

Tung Oil Content (%)	Adhesion Level (Level)
0%	1
5%	2
15%	3
30%	5

). Lacquerware is often a combination of the base material and raw lacquer, so the adhesion strength of the lacquer layer largely determines both its production efficiency and service life. Given the low roughness of glass slides, adhesion to raw lacquer is suboptimal. Therefore, Prunus serotina was employed in this study, which offers improved adhesion to the raw lacquer as the substrate. The data show that without tung oil, the raw lacquer achieved the highest adhesion level (level 1), with the film peeling confined to less than 5% of the cross-cut area. The introduction of tung oil resulted in a reduction in adhesion: it decreased to level 2 (peeling between 5% and 15%) with the addition of tung oil, to level 3 with 15% tung oil, and reached level 5 with a 30% concentration, where significant peeling occurred. These results, which deviated from some previous reports, possibly due to differences in substrates, indicated that increased tung oil content adversely affects the adhesion of the raw lacquer coating film. When the addition level reached 30%, the adhesion of the coating film was significantly weakened, dropping to a level of 5. This could have considerable negative impacts on the production and use of lacquerware.

3.6. Evaluation of Coating Film Mechanical Engraving Performance

Based on the evaluation of the mechanical properties of coating films with varying tung oil concentrations, particularly focusing on film adhesion and drying time, three samples were produced for mechanical engraving testing. These samples were coated with raw lacquer containing 5% and 15% tung oil, respectively. The choice of square and circular patterns was made to simulate the effects of straight lines and curves on the carving results in actual carving production. The wooden blocks were placed on a 3D CNC machine, and a pre-prepared engraving process was loaded into the system. After engraving, the three wooden blocks were compared to assess the performance of the coating films (see Figure 2).

The visual inspection of the engraving results with a magnifying glass revealed that the lacquer without tung oil had smooth edges but exhibited unevenness, likely due to the high hardness and adhesion of the coating film. With 5% tung oil, the engraving edges were smooth, with consistent depression depths, resulting in better overall engraving quality. In contrast, with 15% tung oil, the edges were rough, with noticeable burrs and unevenness, likely due to decreased hardness and adhesion of the coating film.

3.7. SEM Analysis of Coating Film Cross-Sections

The cross-sections of the coating films subjected to mechanical engraving were analyzed using scanning electron microscopy (SEM), as shown in Figure 3. To better illustrate the point, the main observation area has been marked in red. The images reveal that, without tung oil, the coating film exhibited prominent tool marks on the cross-section after engraving, with noticeable irregular depressions and debris. At 50 µm magnification, the tool path marks appear relatively rough. This was attributed to the rigid benzene ring structures in the lacquer phenol and the formation of a stable interpenetrating network polymer during the auto-oxidation cross-linking phase, which increased hardness but reduced flexibility, leading to chipping and detachment of the coating film during the engraving process. When the tung oil content was increased to 5%, the tool marks remained evident, but the cross-section of the coating film was noticeably smoother, with fewer and shallower irregular depressions. Further increasing the tung oil content to 15% enhanced the flatness of the cross-section, with a smoother cut surface and relatively smooth tool marks, showing minimal peeling and fine debris. The incorporation of tung oil, with its long-chain structures, significantly improved the impact resistance and flexibility of the modified lacquer composites.

3.8. FT-IR Analysis of Coating Films

For the purpose of facilitating observation, the principal area of peak variation has been designated with color markings. The FT-IR spectra (Figure 4) of sample A (raw lacquer) reveal a peak at 3650.9 cm⁻¹, corresponding to the O−H stretching vibration, indicative of free hydroxyl groups. The peak at 2970.7 cm⁻¹ was attributed to the C−H stretching vibration within the hydrocarbon chain, originating from the long-chain hydrocarbons in the phenolic structure of the lacquer. Additionally, a peak at 1655.6 cm⁻¹ denotes C=C stretching vibration, while the peak at 1405.5 cm⁻¹ was associated with C−H bending vibration. The peak at 1055.7 cm⁻¹ corresponded to C−O stretching vibrations from the hydroxyl and C−O bonds in the lacquer’s phenolic structure [38].

In the FT-IR spectra of sample D (refined tung oil), the peak at 2909.5 cm⁻¹ is due to C−H stretching vibrations from methyl and methylene groups in the triglycerides’ hydrocarbon chains. The peak at 1737.8 cm⁻¹ reflected ester (C=O) stretching vibrations, resulting from the multiple ester bonds present in the triglycerides. The peak at 1437 cm⁻¹ signified C−H bending vibrations, and the peak at 1148.4 cm⁻¹ represented C−O stretching vibrations from ester bonds in the triglycerides. The peak at 978.8 cm⁻¹ corresponded to asymmetric C−H bending vibrations in olefins [39].

For sample C (raw lacquer with 30% refined tung oil), the peak at 3687.7 cm⁻¹ reflected an increase in O−H stretching vibrations [40], suggesting a higher concentration of hydroxyl groups from both lacquer and tung oil, potentially due to hydrogen bonding or retention of free states. The peak at 2954.9 cm⁻¹ showed intensified C−H stretching vibrations [41], attributed to the additional long−chain fatty acids from the tung oil. The peak at 1403.8 cm⁻¹ corresponded to the bending vibration of the C−H bond. Due to the higher aliphatic chain content in tung oil, the absorption intensity in this region is increased compared to the unmodified lacquer. The peak at 1244.6 cm⁻¹ represents enhanced C−O stretching vibrations from ester compounds [42]. The peak at 1059.2 cm⁻¹ corresponds to the stretching vibration of the C−O bond [43]. Compared to the unmodified coating film, the enhancement of this peak may be attributed to the interaction between ester bonds in the refined tung oil and hydroxyl groups in the raw lacquer.

The incorporation of 30% refined tung oil results in increased quantities of hydrocarbon chains (−CH₂, −CH₃), hydroxyl groups (−OH), and ester bonds (C−O), as evidenced by enhanced absorption peaks. This increase is likely attributed to physical mixing and hydrogen bonding. While the primary interaction was physical, minor chemical reactions, such as esterification or alcoholysis between the tung oil and the lacquer components, may occur. The disappearance of the peak at 1655.6 cm⁻¹ in natural lacquer after the addition of refined tung oil may be attributed to the reaction of the C=C double bonds in the lacquer with components in the tung oil [44]. This could involve addition or cross-linking reactions, which lead to a reduction or disappearance of the double bonds [45]. Alternatively, the introduction of refined tung oil might alter the hydrogen bonding and van der Waals interactions between lacquer molecules, resulting in changes to the infrared absorption of the C=C bonds, which may directly result in changes in the physical properties of the coating film, such as hardness.

4. Conclusions

This study investigated the effects of tung oil addition on the lacquer film and the performance of mechanically carved lacquerware. The results indicate that increased tung oil content prolongs the drying time of the lacquer film. Additionally, the film’s roughness, as measured by R_a and R_q, decreased with higher tung oil concentrations. The 15% tung oil concentration yielded the highest gloss, with other concentrations showing reduced glossiness. The increased tung oil content resulted in decreased film hardness. Moreover, the increase in tung oil content can lead to a decrease in the adhesion strength of the lacquer film. The optimal amount of tung oil improved the mechanical carving quality of the lacquer film, increasing the smoothness and cutting efficiency of the carved surfaces. FT−IR analysis suggested that differences between natural raw lacquer and refined tung oil primarily arise from physical mixing and hydrogen bonding interactions that alter functional group quantities.

Overall, the mixture of raw lacquer with a 15% tung oil content demonstrated a relatively balanced performance in the simulated production of mechanically carved lacquerware during this experiment. The addition of refined tung oil to raw lacquer significantly affects several key performance metrics for mechanical carving, thereby supporting the viability of mechanical carving methods. Nevertheless, this study has not determined the optimal tung oil concentration for mechanical carving or evaluated other carving techniques, such as laser engraving. Additionally, given the observed reduction in certain properties of the coating film after tung oil addition, future research may explore the potential of combining UV−curable materials or other polymer substances to enhance adhesion properties and other mechanical performance characteristics with the substrate. Further investigation is also needed to address other issues related to raw lacquer films during the carving process, such as high viscosity, application difficulties, and challenges in spraying.