1 Material Positioning of Natural Lacquer

1.1 Natural lacquer is not an ordinary natural varnish

Natural lacquer generally refers to a coating material formed from lacquer sap collected from lacquer-tree species after refining and application. Traditional East Asian lacquer is mainly obtained from the lacquer tree, and its sap contains urushiol, water, plant gums, nitrogen-containing substances, laccase, and small amounts of other components. Unlike natural varnishes such as dammar resin, mastic resin, and shellac, which mainly form films through solvent evaporation, the core feature of natural lacquer is that it forms a dense and durable lacquer film through processes such as oxidative polymerization of urushiol, free-radical coupling, side-chain oxidation, and subsequent crosslinking.

In this article, “natural lacquer” mainly refers to urushiol-type natural lacquer represented by traditional Chinese, Japanese, and Korean lacquer. In a broader sense, Oriental lacquer also includes natural lacquer systems in which laccol, thitsiol, and other catechol lipid components are the main film-forming substances. Their film-forming mechanisms are similar, but their specific compositions differ.

1.2 Main value of natural lacquer in coatings

Natural lacquer has long been used in traditional objects, woodware, handicrafts, and protective decorative coatings. Its value mainly comes from the urushiol structure and the dense film formed through oxidative polymerization.

1.3 Core issues of natural lacquer

Natural lacquer has excellent coating film properties, but it is not a general-purpose coating that is easy to industrialize. Its application is limited by curing conditions, application techniques, raw material costs, and safety concerns.

2 Why the Structure of Urushiol Determines the Performance of Natural Lacquer

2.1 Urushiol is the main film-forming substance in urushiol-type natural lacquer

In urushiol-type natural lacquer, urushiol is the key film-forming component. It belongs to the catechol class of compounds. Its molecule contains two adjacent hydroxyl groups on the benzene ring and a relatively long unsaturated alkyl side chain. This structure gives urushiol both strong reactivity and a certain degree of hydrophobicity.

① Catechol structure: provides oxidative reaction sites, readily forms free radicals, and participates in polymerization.

② Long unsaturated side chain: provides hydrophobicity, flexibility, and the possibility of subsequent oxidative crosslinking.

2.2 Catechol structure and free-radical formation

The catechol structure in urushiol molecules is the key reason why natural lacquer can undergo oxidative polymerization. The catechol structure is readily oxidized to form phenoxy radicals, which then undergo coupling reactions and gradually form dimers, oligomers, and higher-molecular-weight structures.

This structural feature explains the fundamental difference between natural lacquer and ordinary volatile natural varnishes. Materials such as dammar resin, mastic resin, and shellac mainly rely on resin deposition after solvent evaporation to form a film, whereas the urushiol in natural lacquer can further undergo oxidative polymerization to form a more stable coating film structure.

2.3 Long unsaturated side chains and subsequent crosslinking

The long unsaturated side chain in urushiol molecules is also important for coating film performance. It not only provides hydrophobicity, but also participates in crosslinking reactions during subsequent oxidation, allowing the lacquer film to gradually form a denser and more stable structure. The effects of the side-chain structure mainly include:

① Enhanced hydrophobicity: reduces the rapid penetration of water into the coating film.

② Flexibility: prevents the coating film from becoming completely rigid and brittle.

③ Participation in post-curing: unsaturated bonds can continue to undergo oxidation and crosslinking.

④ Influence on hardness development: as oxidative crosslinking proceeds, the coating film gradually becomes harder.

⑤ Influence on long-term performance: the degree of side-chain oxidation and crosslinking affects final durability.

3 Oxidative Polymerization Film Formation of Natural Lacquer

3.1 Curing of natural lacquer is not simple evaporation drying

Natural lacquer sap contains urushiol, water, laccase, plant gums, nitrogen-containing substances, and other components. After application, the coating film does not form a simple resin film through rapid solvent evaporation. Instead, oxidative polymerization occurs under suitable temperature, humidity, and oxygen conditions. This film-forming method gives natural lacquer good durability, but it also means that curing is relatively slow and that application conditions are demanding.

From the perspective of phase structure, raw lacquer can usually be regarded as a high-viscosity water-in-oil natural emulsion, with urushiol as the continuous phase and water as the dispersed phase. Laccase, glycoproteins, and other copper-containing proteins participate near the phase interface in free-radical generation, radical transfer, and regulation of polymerization.

3.2 Laccase-catalyzed stage and application conditions

During the initial curing of natural lacquer, laccase-catalyzed oxidation plays an important role. Laccase is an important oxidase involved in the curing of natural lacquer. In the presence of water and oxygen, laccase promotes the oxidation of urushiol, driving the coating film to gradually change from a liquid state to a gel state and then to a solid state.

Whether laccase catalysis proceeds smoothly is closely related to application conditions. Traditional natural lacquer curing usually requires relatively high humidity and a suitable temperature. Common conditions are about 20–30 °C and 70–80% RH, where RH means relative humidity. Surface drying or a workable state may appear earlier, but complete curing and the stabilization of properties such as hardness and water resistance often require several days to several weeks, depending on lacquer sap activity, film thickness, and the application environment.

If the temperature is too low, humidity is insufficient, oxygen supply is inadequate, the film layer is too thick, or improper storage of lacquer sap reduces enzyme activity, natural lacquer may show slow drying, a tacky surface, uneven film formation, or insufficient curing.

3.3 Autoxidation and post-curing stage

Laccase-catalyzed oxidation mainly drives initial polymerization, but the final formation of a durable natural lacquer film also depends on subsequent autoxidation and side-chain crosslinking. The unsaturated structures in the urushiol side chains can continue to participate in oxidation reactions, increasing molecular weight and making the network structure denser. The curing process of natural lacquer can be divided into three interconnected stages:

This is also why natural lacquer films often require a long time to reach stable performance. Initial surface drying does not mean that the interior has completely cured.

3.4 Effects of temperature and humidity on curing quality

Natural lacquer curing is sensitive to temperature and humidity because initial curing depends on laccase activity, while subsequent curing requires oxygen to participate in continuous oxidation. Temperature, humidity, oxygen diffusion, and film thickness jointly determine whether curing is uniform.

When humidity is too low, laccase activity cannot be fully expressed, and initial polymerization is hindered. When humidity is too high, the coating surface may become hazy, lose gloss, or dry unevenly. Low temperature reduces the reaction rate, while excessive film thickness limits oxygen penetration, causing the inner and outer layers to cure asynchronously. Therefore, natural lacquer application is not merely a coating process; it is also a process of controlling the curing environment and film thickness.

4 Why Natural Lacquer Coating Films Are Durable

4.1 Crosslinked structure improves coating film stability

The lacquer film formed after natural lacquer cures is not a simple deposited film. Rather, it is a network structure formed by oxidative polymerization of urushiol and side-chain crosslinking. This structure gives the lacquer film good mechanical stability, water resistance, and long-term durability. The effects of the crosslinked structure on the coating film include:

① Reducing molecular chain mobility and improving film stability.

② Reducing the entry of water and small molecules into the coating film.

③ Increasing film hardness and scratch resistance.

④ Enhancing the protective effect of the coating film on the substrate.

⑤ After complete curing, improving resistance to water and some common solvents. However, this does not mean the film is stable against all organic solvents, strong acids, strong alkalis, or long-term immersion environments.

The durability of natural lacquer comes from its crosslinked structure, not simply from film thickness. Thick application does not necessarily improve performance; instead, it may cause uneven curing between the inner and outer layers.

4.2 Hydrophobic side chains improve water resistance

The long alkyl chains of urushiol are strongly hydrophobic and can reduce the rapid penetration of water into the lacquer film. As subsequent oxidative crosslinking proceeds, the lacquer film structure becomes denser, and water resistance is further enhanced.

Good water resistance of natural lacquer does not mean that the highest level of water resistance is achieved immediately after application. The natural lacquer coating film must be fully cured before its water resistance, hardness, and durability gradually stabilize.

4.3 Good adhesion comes from both structure and application

Natural lacquer shows good adhesion to wood, bamboo, fabric, metal substrates, and other materials, but adhesion does not come only from the urushiol structure. It is also closely related to substrate treatment and application technique.

4.4 Decorative properties come from gloss, color depth, and film texture

Natural lacquer has a distinctive decorative effect. After complete curing, sanding, and polishing, the lacquer film can show a deep, warm, and refined surface. Its decorative quality mainly comes from film density, surface flatness, light reflection, and the combined color expression of pigments or the substrate. Natural lacquer can form either a high-gloss surface or a softer semi-gloss or matte surface. Through multiple rounds of application, sanding, lacquering, and polishing, natural lacquer can achieve different layers of surface effect. This decorative quality is an important feature that distinguishes natural lacquer from ordinary protective coatings.

5 Typical Applications of Natural Lacquer

5.1 Lacquerware and traditional craft coatings

Typical applications of natural lacquer include lacquerware and traditional craft coatings. Substrates such as wood, bamboo, cloth cores, and paper cores can form lacquer films with both decorative and protective functions after priming, filling, lacquering, drying, sanding, and polishing.

5.2 Wood and bamboo coating

Natural lacquer is well suited to wood and bamboo. Its lacquer film can improve the water resistance, stain resistance, and decorative effect of the substrate surface. For high-value woodware, traditional furniture, and bamboo or wood handicrafts, natural lacquer still has unique value. When using natural lacquer for wood and bamboo coating, attention should be paid to the following factors:

① The moisture content of the substrate should be appropriate.

② The surface should be thoroughly sanded and cleaned.

③ Coatings should be applied in multiple thin layers.

④ Each coat should cure under suitable temperature and humidity.

⑤ Polishing or subsequent treatment should be carried out only after complete curing.

Natural lacquer is not suitable for ordinary wood coating that pursues rapid, large-scale application. It is more suitable for products that emphasize durability, traditional texture, and high decorative quality.

5.3 Protective decorative coatings

Natural lacquer can be used as a coating that combines protection and decoration, such as on the surfaces of traditional objects, artworks, special decorative panels, and high-value handmade products. Its advantages lie in its dense coating film, good water resistance, and distinctive surface texture. However, when natural lacquer is used as a protective decorative coating, two misunderstandings should be avoided:

① Do not focus only on durability while ignoring application conditions.

Natural lacquer requires suitable temperature and humidity as well as sufficient curing time.

② Do not equate natural lacquer with modern heavy-duty anticorrosive coatings.

Natural lacquer has good traditional protective performance, but in high-salt-spray, strongly acidic or alkaline, strong ultraviolet, or heavy industrial corrosive environments, it still needs to be tested according to actual service conditions.

6 Limitations and Use Risks of Natural Lacquer

6.1 Application limitations caused by slow curing

The oxidative polymerization and subsequent crosslinking processes of natural lacquer are relatively slow. Even when the surface is no longer tacky, the internal structure may still be curing. If temperature and humidity are unsuitable, the film layer is too thick, or the lacquer sap has insufficient activity, curing time will be further prolonged, causing surface tackiness, an uncured inner layer, slow hardness development, or insufficient water resistance. Slow curing is an important limitation that distinguishes natural lacquer from many modern industrial coatings. It is not suitable for coating scenarios that require fast drying, rapid packaging, quick stacking, or short-cycle production-line application.

6.2 High requirements for application conditions

Natural lacquer application requires relatively strict environmental control. Traditional application often needs to be carried out in a lacquer room or humidity-controlled environment to maintain suitable temperature and humidity. If application conditions are not properly controlled, the appearance and final performance of the film may be affected. Common application problems include excessively slow drying, surface tackiness, loss of gloss or hazing, wrinkling or sagging, poor interlayer bonding, and insufficient internal curing. These problems are usually related to film thickness, ambient humidity, ventilation, lacquer sap condition, and application experience.

6.3 Allergy and safety risks

Urushiol is highly reactive and may also cause contact dermatitis. Uncured natural lacquer poses a relatively high risk of skin irritation and sensitization, so applicators need to take protective measures. After natural lacquer is completely cured, the risk is significantly reduced, but safety protection is still important during application and restoration.

6.4 Cost and supply limitations

The collection and refining of natural lacquer depend on manual labor and producing-area conditions. High-quality lacquer sap is collected in limited quantities, has a relatively high cost, and is significantly affected by tree species, season, production area, storage method, and refining process. As a result, the application of natural lacquer in modern coatings is mainly concentrated in high-value products, traditional crafts, art restoration, and special decorative fields, rather than ordinary low-cost coating.

6.5 Unsuitable application scenarios

7. Representative Chemical Products Related to the Durability Mechanism of Natural Lacquer

Table 1. Products Related to Urushiol Structure, Enzymatic Oxidation, and Free-Radical Polymerization Research

Table 2. Products Related to Buffer Systems, Acid–Base Environments, and Coating Film Resistance Evaluation

Table 3. Products Related to Coating Film Solvent Resistance, Hydrophobic Structure, and Film-Forming Performance Research

Note: The above are representative Aladdin products. More product specifications can be searched on the Aladdin official website by product name, CAS number, or catalog number.

References

[1] Lu R., Kamiya Y., Miyakoshi T. Applied Analysis of Lacquer Films Based on Pyrolysis-Gas Chromatography/Mass Spectrometry. Talanta, 2006, 70(2), 370–376.

[2] Yang J., Chen N., Zhu J., et al. Polymerization mechanism of natural lacquer sap with special phase structure. Scientific Reports, 2020, 10, 12867.

[3] Chen Y., et al. Catechol Derivative Urushiol’s Reactivity and Applications Beyond Coating. Industrial Crops and Products, 2023.

[4] Advances in the Chemical Properties and Functional Applications of Urushiol: From Traditional Lacquerware to Modern Materials. Polymers, 2026, 18(9), 1072.

[5] Research on the Rapid Curing Mechanism and Technology of Chinese Lacquer. Polymers, 2025, 17(12), 1596.

[6] Pang L., Yang X., Gu J. Investigation on the enhanced process and mechanism of laccase-like Cu-MOF-induced curing of raw lacquer. Journal of Coatings Technology and Research, 2026, 23, 445–465. Published online 3 Oct 2025.

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