Natural Resins in Coating Formulations: Functional Positioning, Structure–Property Relationships, and Selection Validation
Natural Resins in Coating Formulations: Functional Positioning, Structure–Property Relationships, and Selection Validation
1 What Are Natural Resins?
1.1 Basic Concept of Natural Resins
In coating formulations, resins typically serve functions such as film formation, binding, adhesion, protection, and decoration. In its description of “resin,” the International Union of Pure and Applied Chemistry (IUPAC) notes that the term “resin” was originally used because of similarities to natural resins, such as rosin, and was later extended to refer to base materials for plastics, organic coatings, and varnishes.
Natural resins generally refer to a class of organic materials derived from natural resources, possessing resinous characteristics, and usable for film formation or performance modification in coatings. Common natural resins include rosin, shellac, dammar resin, copal resin, mastic resin, sandarac, Canada balsam, natural lacquer, and others.
A natural resin is not a single chemical substance with a fixed structure, but rather a broad category of materials that differ significantly in source, composition, and performance. Therefore, when determining whether a natural resin is suitable for a coating formulation, one should not only consider whether it is “natural,” but also what role it plays in the formulation and whether it can meet the performance requirements of the coating film.
1.2 Differences Between Natural Resins, Natural Oils and Fats, Bio-Based Resins, and Natural Polymers
Natural resins are often mistakenly understood as all coating raw materials derived from natural sources. In practical material selection, natural resins, natural oils and fats, bio-based resins, and natural polymers should be distinguished from one another.
Category | Typical Examples | Main Functions in Coatings | Difference from Natural Resins |
Natural resins | Rosin, shellac, dammar resin, copal resin, natural lacquer | Film formation, hardness enhancement, gloss enhancement, adhesion improvement, drying adjustment, participation in modification | Possess resinous characteristics, film-forming ability, or binding function |
Natural oils and fats | Tung oil, linseed oil, soybean oil, castor oil | Oxidative drying, alkyd resin synthesis, flexibility improvement | The main components are usually triglycerides or fatty acids; they are not equivalent to natural resins |
Bio-based resins | Bio-based epoxy resins, bio-based polyurethane resins, bio-based acrylic resins | Main film-forming resins or reactive resins | Emphasize that the raw materials contain renewable components; they may undergo extensive chemical synthesis |
Natural polymers | Cellulose, starch, lignin, chitosan | Thickening, film formation, dispersion, functional modification | Most are natural polymer materials and are not traditional natural resins |
Bio-based coatings are a broader concept and usually include bio-based resins, pigments and fillers, additives, solvents, and other components. Natural resins can be part of the raw materials used in bio-based coatings, but not all bio-based raw materials should be called natural resins.
Natural-source raw materials such as Cashew Nut Shell Liquid, or CNSL, are also commonly used for coating resin modification. However, CNSL is more appropriately regarded as a reactive raw material of natural origin, rather than being simply grouped together with traditional natural resins such as rosin, shellac, and dammar resin.
1.3 Why Natural Resins Should Be Understood from the Perspective of Formulation Function
When natural resins are used in coatings, the most important question is not “what kind of natural material is it,” but “what problem does it solve in the formulation?” The same natural resin may play different roles in different systems. For example, it may serve as the main film-forming material in certain varnishes, as an auxiliary resin in synthetic resin systems, or as a reactive resin raw material after chemical modification. Therefore, the selection of natural resins should first answer three questions:
Evaluation Question | Specific Meaning |
What role does it play in the formulation? | Main film formation, auxiliary modification, performance adjustment, or special function |
Can its structure explain the target performance? | Whether it has rigid structures, polar groups, hydrophobic structures, or reactive groups |
Is it suitable for the current coating system? | Whether it dissolves, is compatible and stable, and whether it affects application and coating film performance |
2 Four Roles of Natural Resins in Coatings
2.1 As Main Film-Forming Resins
Some natural resins have good film-forming ability and can form continuous coating films through solvent evaporation or thermoplastic film formation. Special systems such as natural lacquer can also form coating films through oxidative or enzymatic polymerization. When used as the main film-forming resin, the natural resin determines the basic properties of the coating film, including hardness, gloss, adhesion, water resistance, chemical resistance, and appearance.
Item of Concern | Key Evaluation Points |
Film-forming ability | Whether it can form a continuous, uniform coating film without obvious defects |
Adhesion | Whether it can firmly adhere to wood, paper, metal, or other substrate surfaces |
Surface properties | Whether it has appropriate hardness, gloss, feel, and blocking resistance |
Resistance | Whether it can withstand water, alcohol, alkali, heat, light exposure, and other service conditions |
Stability | Whether it is prone to yellowing, embrittlement, tack-back, or aging |
2.2 As Auxiliary Resins
In modern coatings, natural resins are more often used not as standalone main resins, but as auxiliary resins incorporated into formulations to improve specific properties.
Auxiliary Function | Effect on the Coating Film or Application |
Increasing hardness | Improves surface scratch resistance, blocking resistance, and sandability |
Increasing gloss | Improves surface brightness in varnishes, inks, or pigmented coatings |
Improving adhesion | Enhances bonding between the coating film and substrate or between coating layers |
Adjusting drying | Improves surface drying, through drying, recoating, or application rhythm |
Improving pigment wetting | Helps pigment dispersion and coating film appearance |
Adjusting bonding properties | Used in systems related to inks, varnishes, and adhesives |
2.3 As Modification Raw Materials
Natural resins often contain reactive structures such as carboxyl groups, hydroxyl groups, phenolic hydroxyl groups, and unsaturated bonds. Therefore, they can serve as modification raw materials in resin synthesis or resin modification.
Modification Purpose | Potentially Improved Properties |
Improving compatibility | Improves blending performance with alkyd, epoxy, polyurethane, acrylic, and other resins |
Adjusting hardness and flexibility | Improves scratch resistance, blocking resistance, or flexibility of the coating film |
Improving water resistance | Enhances hydrophobicity or reduces water sensitivity |
Increasing reactivity | Participates in crosslinking, curing, or polymerization reactions |
Improving storage stability | Reduces the risk of precipitation, haze, or oxidative deterioration |
Reducing defects from direct use | Lessens the color, odor, brittleness, or insufficient weatherability of the original natural resin |
Modified natural resins are usually more suitable for industrial coating applications than unmodified natural resins. They retain certain structural characteristics of natural origin while improving stability, compatibility, and coating film performance through chemical modification.
2.4 As Special Functional Materials
Some natural resins are not suitable as general-purpose main resins, but they have clear functional value in specific coatings.
Functional Direction | Application Purpose |
Substrate sealing | Seals knots, pigments, tannins, odors, or substrate contaminants in wood |
Decorative enhancement | Enhances transparency, gloss, color enrichment, and surface texture |
Restoration and protection | Used as protective layers in traditional coatings, artworks, or wood restoration |
Bio-based design | Increases the proportion of renewable-source raw materials in the formulation |
Interface adjustment | Improves wetting, adhesion, surface energy, or interlayer bonding between coatings |
Such applications usually do not require natural resins to provide all coating film properties. Instead, they make use of a natural resin’s specific functional advantage. Suitability should be judged by whether the resin can consistently solve a specific problem.
3 Why Natural Resins Can Affect Coating Film Performance
3.1 Rigid Structures and Hardness, Gloss
Many natural resins contain cyclic or fused-ring structures. These structures have relatively low molecular mobility and tend to increase coating film hardness, blocking resistance, and surface gloss.
Structural Feature | Performance Effect |
Cyclic structure | Improves coating film hardness and surface compactness |
Higher softening point | Improves blocking resistance, heat resistance, and sandability |
Good transparency | Helps varnishes and decorative coatings achieve higher gloss |
However, when the proportion of rigid structures is too high, brittleness may increase, impact resistance may decrease, or flexibility may become insufficient. When natural resins are used to increase hardness, changes in flexibility and adhesion should also be monitored.
3.2 Polar Groups and Adhesion, Reactivity
Common polar groups in natural resins include carboxyl groups, hydroxyl groups, and phenolic hydroxyl groups. These groups can strengthen interactions between the resin and substrates, pigments and fillers, or other resins.
Polar Group | Possible Function |
Carboxyl group | Improves adhesion, increases reactivity, and affects acid value and dispersibility |
Hydroxyl group | Participates in hydrogen bonding or further reactions |
Phenolic hydroxyl group | Provides reactivity and improves adhesion and crosslinking potential |
Polar groups are beneficial for adhesion and modification, but excessive polar group content may increase water absorption and affect water resistance, storage stability, or compatibility with low-polarity resins.
3.3 Hydrophobic Structures and Water Resistance, Wetting, Compatibility
Some natural resins contain hydrophobic hydrocarbon chains or hydrophobic backbones, which can improve water resistance, wetting, or compatibility with nonpolar resins.
Role of Hydrophobic Structures | Possible Performance |
Reducing water sensitivity | Reduces water absorption, whitening, or softening of the coating film |
Improving substrate wetting | Helps wet wood, paper, or low-polarity surfaces |
Adjusting compatibility | Improves blending with certain solventborne resins or oil-based systems |
It should be noted that not all natural resins can improve water resistance. Some natural resins or their modified products can improve hydrophobicity, sealing performance, or water resistance, but the final effect depends on the specific resin type, modification method, addition level, and coating system.
3.4 Unsaturated Structures and Modification, Oxidation, Yellowing
Unsaturated bonds are important structures in many natural-source resins or related raw materials. They can provide possibilities for modification and curing, but may also introduce risks of oxidative aging.
Effect of Unsaturated Structures | Specific Performance |
Participation in reactions | Can be used for grafting, addition, and crosslinking; in drying oils, natural lacquer, or specific modified systems, they may also participate in oxidative curing |
Improvement of coating film formation | Helps form a more compact or higher-molecular-weight structure |
Susceptibility to oxidation | Long-term exposure to heat, light, and oxygen may cause yellowing, embrittlement, or performance deterioration |
Effect on storage stability | May cause viscosity increase, color darkening, or risk of gelation |
3.5 Complex Natural Composition and Batch Stability
Natural resins are usually not single compounds, but mixtures of resin acids, esters, terpenes, phenols, or other components. Their complex composition can produce unique film-forming and decorative effects, but it also increases the difficulty of formulation control.
Compositional Feature | Effect on the Formulation |
Multi-component mixture | May create distinctive gloss, feel, and film-forming characteristics |
Differences in origin | Affect color, acid value, softening point, and impurity content |
Differences in processing | Affect decolorization, dewaxing, purity, and solubility |
Changes during storage | May lead to oxidation, caking, thickening, or color darkening |
4 Establishing a Selection Logic Based on Formulation Objectives
4.1 First Clarify the Property to Be Improved
The first step in selecting a natural resin is to clarify what property the formulation truly needs to improve.
Formulation Objective | Key Focus |
Increasing hardness | Softening point, glass transition temperature, or Tg, and blocking resistance |
Increasing gloss | Transparency, compatibility, leveling, and surface condition |
Improving adhesion | Polar groups, wetting, and substrate suitability |
Improving sealing performance | Film compactness and barrier ability against substrate extractives |
Adjusting drying | Solvent release, oxidative or enzymatic reactions, or other curing speeds |
Improving water resistance | Hydrophobic structures, film compactness, and degree of crosslinking |
Increasing the proportion of renewable-source materials | Raw material source, renewable carbon content, and regulatory requirements |
4.2 Then Determine the Role of the Natural Resin
After the target property has been defined, the role of the natural resin in the formulation should be determined.
Formulation Role | Selection Focus | Typical Risks |
Main film-forming resin | Film-forming ability, adhesion, hardness, water resistance, chemical resistance | Insufficient overall performance |
Auxiliary resin | Compatibility, addition level, gloss, hardness, drying speed | Excessive use may cause embrittlement, haze, or tack-back |
Modification raw material | Functional groups, acid value, hydroxyl value, reactivity, color | Difficult reaction control or batch variation |
Special functional material | Sealing ability, transparency, removability, substrate suitability | Narrow application scope |
The same natural resin may play different roles in different formulations. During selection, its role should be defined first, followed by the corresponding evaluation criteria. This avoids evaluating an auxiliary resin according to the standards for a main resin, or evaluating a reactive raw material according to the standards for an auxiliary resin.
4.3 Evaluate Whether It Is Compatible with the Coating System
Whether a natural resin is suitable for a particular formulation depends on how well it matches the coating system.
Coating System | Questions to Evaluate |
Solventborne coatings | Whether it dissolves in the target solvent and whether it affects transparency, gloss, and storage stability |
Alcohol-soluble systems | Whether it is suitable for alcohol solvents and whether drying speed and film formation are stable |
Waterborne coatings | Whether neutralization, emulsification, dispersion, or chemical modification is required |
High-solids systems | Whether it significantly increases viscosity and whether it affects leveling and application |
Reactive coatings | Whether it has functional groups that can participate in reactions and whether it affects curing speed and degree of crosslinking |
Special caution is needed when natural resins are used in waterborne coatings. Many natural resins are inherently hydrophobic. Without appropriate modification or dispersion design, problems such as coarse particle size, dispersion instability, sedimentation, haze, or gloss reduction may occur.
4.4 Determine the Addition Method and Addition Level
Natural resins should not be added at high levels without prior validation. A reasonable approach is to start with a low addition level and observe performance changes through gradient experiments.
Change in Addition Level | Possible Effect |
Low addition level | May improve gloss, adhesion, hardness, or sealing performance, with limited impact on the system |
Moderate addition level | Performance improvement is more obvious, but compatibility and drying changes need attention |
Excessive addition level | May cause embrittlement, haze, tack-back, yellowing, whitening, or precipitation |
The addition method also affects performance. Natural resins can be pre-dissolved, pre-dispersed, premixed with the main resin, or added after chemical modification. The specific method should be determined according to solubility, compatibility, and application requirements.
5 Indicators That Must Be Verified When Selecting Natural Resins
5.1 Raw Material Indicators: Acid Value, Hydroxyl Value, Softening Point, Color, Volatiles
Raw Material Indicator | Effect on Application |
Acid value | Affects reactivity, dispersibility, water resistance, and suitability for metal substrates |
Hydroxyl value | Affects the design of polyurethane, alkyd, or other reactive systems |
Softening point | Affects hardness, blocking resistance, thermal stability, and application performance |
Color | Affects varnishes, light-colored coatings, and high-appearance coatings |
Volatiles | Affect bubbles, odor, storage stability, and application defects |
Ash and impurities | Affect transparency, filterability, gloss, and coating film appearance |
Batch consistency | Affects continuous production and formulation reproducibility |
5.2 Formulation Compatibility: Solubility, Compatibility, Viscosity, Storage Stability
Whether a natural resin can be incorporated into a coating formulation first depends on formulation compatibility. Compatibility testing should include observation at room temperature and after heat storage. Short-term transparency alone does not indicate long-term stability.
Compatibility Item | Observation Focus |
Solubility | Whether it dissolves completely and whether haze, particles, or sediment appear |
Compatibility | Whether it can coexist stably with the main resin, solvent, additives, pigments, and fillers |
Viscosity change | Whether it causes significant thickening, abnormal thixotropy, or application difficulty |
Storage stability | Whether phase separation, sedimentation, precipitation, caking, or color darkening occurs |
Pigment compatibility | Whether it affects pigment wetting, dispersion fineness, and color paste stability |
5.3 Coating Film Properties: Drying, Hardness, Adhesion, Gloss, Water Resistance, Alcohol Resistance, Yellowing Resistance
Whether a natural resin is ultimately usable should be judged by coating film performance.
Coating Film Property | Evaluation Significance |
Drying performance | Evaluates surface drying, through drying, recoating, and tack-back risks |
Hardness | Evaluates scratch resistance, blocking resistance, and sandability |
Adhesion | Evaluates substrate suitability and interlayer bonding |
Gloss | Evaluates compatibility, leveling, and surface appearance |
Transparency | Evaluates suitability for varnishes, wood coatings, and protective coatings |
Water resistance | Evaluates risks of whitening, blistering, softening, or adhesion loss |
Alcohol resistance | Evaluates suitability for furniture, packaging, wood coatings, or everyday contact scenarios |
Yellowing resistance | Evaluates long-term appearance stability |
Flexibility | Evaluates whether the coating film becomes brittle due to hardness enhancement |
Coating film performance should not be judged by a single indicator. A common issue with natural resins is that one property improves while another declines. For example, hardness may increase while flexibility decreases; gloss may improve while yellowing resistance becomes worse; sealing performance may improve while alkali resistance remains insufficient.
5.4 Use Risks: Embrittlement, Yellowing, Haze, Tack-Back, Whitening, Precipitation
The following risks should be closely monitored when using natural resins.
Use Risk | Possible Cause | Resulting Effect |
Embrittlement | High proportion of rigid structures; excessive addition level | Cracking after bending, impact, or hot–cold cycling |
Yellowing | Oxidation of unsaturated structures or natural impurities | Deteriorated appearance in light-colored coatings and varnishes |
Haze | Insufficient compatibility with the main resin or solvent | Reduced gloss and poorer transparency |
Tack-back | Low softening point, insufficient drying, or excessive addition level | Poor blocking resistance and surfaces prone to contamination |
Whitening | Water absorption, phase separation, or insufficient film compactness | Reduced water resistance and poorer appearance |
Precipitation | Compatibility changes during storage or temperature fluctuations | Sediment, particles, and coating film defects |
Abnormal viscosity | Resin swelling, reaction, or dispersion instability | Application difficulty and reduced storage stability |
These risks do not mean that natural resins cannot be used. Rather, they indicate that addition level, modification method, solvent selection, matching with the main resin, and coating film testing must be used to control such risks.
6 Summary: The Value of Natural Resins Lies in Rational Use After Their Function Is Clearly Defined
The value of natural resins in coatings lies in using their structural characteristics to provide film formation, hardness enhancement, gloss enhancement, adhesion improvement, substrate sealing, drying adjustment, or participation in resin modification.
The selection of natural resins should follow three judgments:
① Natural resins are not universal replacement resins.
Different natural resins differ significantly in structure, composition, and performance. They cannot simply replace synthetic resins according to a single standard.
② Natural resins should be selected according to their formulation role.
Natural resins can serve as main film-forming resins, auxiliary resins, modification raw materials, or special functional materials. Different roles require different evaluation indicators.
③ Whether they are suitable for use ultimately depends on structure, system compatibility, and coating film validation.
Natural origin does not automatically mean high performance, low risk, or high stability. Only after validation of solubility, compatibility, addition level, storage stability, and coating film performance can one determine whether a natural resin is suitable for a specific coating formulation.
7. Representative Product Table of Natural Resins and Related Natural-Source Raw Materials in Coating Formulations
Table 1 Natural Polymers and Related Raw Materials for Bio-Based Coating Research
Category | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
Natural polymer film-forming and thickening material | 9012-76-4 | Chitosan | Medium viscosity, 200–400 mPa·s | Can be used for research on natural polymer film formation, coating modification, antibacterial coatings, and functionalization of waterborne systems | |
Cellulose ether thickener | 9004-32-4 | Sodium carboxymethyl cellulose, CMC | Viscosity: 1000–1400 mPa·s, USP grade | Can be used for research on thickening, water retention, rheology control, pigment and filler suspension in waterborne coatings, and natural polymer formulations | |
Starch-based natural polymer | 9005-25-8 | S116028 | Corn starch | Pharmaceutical grade, PharmPure™ | Can be used for research on natural polymer film formation, degradable coatings, bonding systems, and bio-based coating formulations |
Cellulose nanomaterial | 9004-34-6 | Cellulose | Nanocrystals, L: ~200 nm, OD: ~10 nm | Can be used for research on waterborne coating reinforcement, barrier coatings, rheology control, nanocomposite coating films, and bio-based materials | |
Cellulose ether thickener | 9004-62-0 | 2-Hydroxyethyl cellulose, HEC | Average Mw ~380,000 | Can be used for research on thickening, leveling control, storage stability improvement in waterborne coatings, and latex paint formulations | |
Lignin-based natural polymer | 9005-53-2 | Lignin, dealkalized | — | Can be used for research on bio-based resin modification, UV-absorbing coatings, antioxidant coatings, and phenolic resin replacement |
Table 2 Natural Oils and Oxidative-Drying Coating Raw Materials
Category | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
Natural semi-drying oil | 8001-22-7 | Soybean oil | Pharmaceutical grade, PharmPure™ | Can be used for research on alkyd resins, bio-based coating resins, oil-based coatings, and renewable fatty acid systems | |
Natural non-drying oil / hydroxyl oil | 8001-79-4 | C434218 | Castor oil | European Pharmacopoeia, Ph. Eur. | Can be used for research on polyurethane polyols, flexible coatings, alkyd modification, and bio-based resin synthesis |
Natural drying oil | 8001-26-1 | Linseed oil | ≥99% | Can be used for research on oxidative-drying coating films, oil-based varnishes, alkyd resins, traditional coatings, and drying-oil curing | |
Natural drying oil | 8001-20-5 | Tung oil | — | Can be used for research on fast-drying oil-based coatings, alkyd modification, water-resistant coatings, traditional varnishes, and oxidative crosslinked film formation |
Table 3 Natural Resins, Natural Balsams, and Natural-Source Phenolic Structural Raw Materials
Category | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
Natural resin acid-based resin | 8050-09-7 | Rosin | Natural resin | Can be used for research on natural resin structures, hardness and gloss enhancement in coatings, adhesion improvement, ink vehicles, and rosin-modified resins | |
Traditional natural balsam resin | 8007-47-4 | Canada balsam | BioReagent | Can be used for optical mounting, bonding of mineral or biological sections, and research on traditional natural resin varnish materials and resin transparency | |
Natural-source phenolic structural reference material | 501-24-6 | 3-Pentadecylphenol | ≥90% mixture of isomers | Can be used for research on cardanol structures, hydrophobic phenolic modification, epoxy resin modification, and bio-based phenolic resins |
Note: The products listed above are representative Aladdin products. For more product specifications, please search by “product name / CAS / catalog number” on the Aladdin official website.
References
[1] IUPAC. Compendium of Chemical Terminology: Resin. IUPAC Gold Book.
[2] Bio-Based Coatings: Progress, Challenges and Future Perspectives. Polymers, 2025.
[3] RSC. “Rosin and Other Natural Resins and Gums.” In: Renewable Resources for Functional Polymers and Biomaterials.
[4] SpecialChem. Natural Resins in Coatings and Inks: Overview and Benefits.
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