The curing mechanism of epoxy resins and commonly used curing agents.
Epoxy resin, as an important polymer material, is widely used in various fields due to its unique properties. This article will provide a comprehensive analysis of epoxy resin, covering its basic characteristics, production process, usage methods, performance optimization, application areas, and safety and environmental aspects.
Performance characteristics of epoxy resins and their cured products
High mechanical properties. Epoxy resin has strong cohesive force and a dense molecular structure, so its mechanical properties are superior to general-purpose thermosetting resins such as phenolic resin and unsaturated polyester. 2. Strong adhesion. The epoxy resin curing system contains highly active epoxy groups, hydroxyl groups, and polar groups such as ether bonds, amine bonds, and ester bonds, giving epoxy cured products excellent adhesion to polar substrates such as metals, ceramics, glass, concrete, and wood. 3. Low curing shrinkage rate. Generally 1% to 2%. It is one of the thermosetting resins with the lowest curing shrinkage rate (phenolic resin is 8% to 10%; unsaturated polyester resin is 4% to 6%; silicone resin is 4% to 8%). The linear expansion coefficient is also very small, generally 6 × 10⁻⁵/℃. Therefore, the volume change after curing is small. 4. Good processability. Epoxy resin basically does not produce low-molecular-weight volatile substances during curing, so it can be molded under low pressure or contact pressure. It can be combined with various curing agents to manufacture environmentally friendly coatings such as solvent-free, high-solid, powder coatings, and water-based coatings. 5. Excellent electrical insulation. Epoxy resin is one of the thermosetting resins with the best dielectric properties. 6. Good stability and excellent chemical resistance. Epoxy resin free of impurities such as alkalis and salts is not easily deteriorated. As long as it is stored properly (sealed, protected from moisture, and not exposed to high temperatures), its storage period is 1 year. If it passes inspection after the expiration date, it can still be used. Epoxy cured products have excellent chemical stability. Its resistance to corrosion from various media such as alkalis, acids, and salts is superior to thermosetting resins such as unsaturated polyester resin and phenolic resin. Therefore, epoxy resin is widely used as an anti-corrosion primer, and because the epoxy cured product has a three-dimensional network structure and can withstand immersion in oils, it is widely used in oil tanks, oil tankers, and the inner lining of integral fuel tanks of aircraft. 7. The heat resistance of epoxy cured products is generally 80–100°C. Heat-resistant varieties of epoxy resins can withstand temperatures of 200°C or higher. However, epoxy resins also have some drawbacks, such as poor weather resistance. Epoxy resins generally contain aromatic ether bonds, and the cured material is prone to degradation and chain scission when exposed to sunlight. Therefore, typical bisphenol A type epoxy resin coatings easily lose their gloss and gradually become chalky when exposed to sunlight outdoors, making them unsuitable for use as outdoor topcoats. Additionally, epoxy resins have poor low-temperature curing performance; they generally require temperatures above 10°C for curing, and curing is slow below 10°C. This makes construction in cold weather very inconvenient for large structures such as ships, bridges, harbors, and oil tanks.
A Brief History of Epoxy Resin Development
The research on epoxy resins began in the 1930s: In 1934, P. Schlack of I.G. Farben in Germany discovered that amine compounds could polymerize compounds containing multiple epoxy groups into high-molecular-weight compounds, producing plastics with low shrinkage, thus obtaining a German patent. Shortly afterward, Pierre Castan of Gebr. de Trey in Switzerland and S.O. Greelee of Devoe & Raynolds in the United States synthesized epoxy resins through polycondensation of bisphenol A and epichlorohydrin. The resins could be cured with organic polyamines or phthalic anhydride and exhibited excellent adhesive properties. Soon after, Ciba in Switzerland, Shell in the United States, and Dow Chemical also began industrial production and application development research of epoxy resins. In the 1950s, while the production and application of ordinary bisphenol A epoxy resins continued, several new types of epoxy resins emerged. Around 1960, thermoplastic phenolic epoxy resins, halogenated epoxy resins, and polyolefin epoxy resins appeared. China began developing epoxy resins in 1956, with initial success in Shenyang and Shanghai. Industrial production began in Shanghai and Wuxi in 1958. In the mid-1960s, research began on several new types of cycloaliphatic epoxies: phenolic epoxy resins, polybutadiene epoxy resins, glycidyl ester epoxy resins, and glycidylamine epoxy resins. By the late 1970s, China had formed a complete industrial system encompassing monomers, resins, auxiliary materials, and covering research, production, and application. In recent years, my country’s development and application research of epoxy resins have progressed rapidly, with continuously increasing production, improved quality, and the emergence of new varieties. Production capacity has increased from less than 15,000 tons per year to approximately 1.44 million tons currently, and is expected to reach around 1.47 million tons by 2024.
Curing agents and curing reactions of epoxy resins
Epoxy resins are inherently stable; for example, bisphenol A-type epoxy resin remains unchanged even when heated to 200°C. However, the epoxy resin molecule contains reactive epoxy groups, making it highly reactive and capable of undergoing a curing reaction with a curing agent to form a network macromolecule. The curing reaction of epoxy resins is mainly related to the epoxy and hydroxyl groups in the molecule. The curing reaction of epoxy resins is achieved by adding a curing agent, utilizing certain groups in the curing agent to react with the epoxy or hydroxyl groups in the epoxy resin. There are many types of curing agents; based on their chemical composition and structure, commonly used curing agents can be classified into amine curing agents, anhydride curing agents, synthetic resin curing agents, and polysulfide rubber curing agents.
Amine curing agents
The amount of amine curing agent used depends on the relative molecular weight of the curing agent, the number of active hydrogen atoms in the molecule, and the epoxy value of the epoxy resin. Amine curing agents include polyamine curing agents, tertiary amines and imidazole curing agents, and boramine and its boramine complex curing agents.
Polyamine curing agents
Single polyamine curing agents include aliphatic polyamine curing agents, polyamide polyamine curing agents, alicyclic polyamine curing agents, aromatic polyamine curing agents, and other amine curing agents.
Aliphatic polyamine curing agents:
These curing agents can cure epoxy resins at room temperature, offering fast curing speed and low viscosity. They can be used to formulate solvent-free or high-solids coatings that cure at room temperature. Commonly used aliphatic polyamine curing agents include ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, hexamethylenediamine, and m-xylylenediamine. Epoxy resins cured with straight-chain aliphatic amines generally exhibit good toughness, excellent adhesion, and excellent resistance to strong alkalis and inorganic acids, but the paint film has poor solvent resistance. Aliphatic polyamine curing agents have the following disadvantages: they generate a large amount of heat during curing, so the amount of paint prepared should not be too large, and the working time is short; they have a very low active hydrogen equivalent, so accurate weighing is essential when preparing the paint, as excess or insufficient amounts will affect performance; they have a certain vapor pressure and are irritating, affecting worker health; they are hygroscopic, making them unsuitable for application at low or high temperatures, and they easily absorb CO2 from the air to form carbamides; they are highly polar, resulting in poor miscibility with epoxy resins, which can easily cause defects such as pinholes, orange peel, and whitening in the paint film.
Polyamide Polyamine Curing Agents:
These are modified polyamines, formed by the condensation of vegetable oil fatty acids with polyamines, containing both amide and amine groups.
Alicyclic Polyamine Curing Agents:
These have a light color, good color retention, and low viscosity, but their reaction is slow. They often need to be used in combination with other curing agents, or with accelerators, or as adducts, or require heat curing.
Aromatic Polyamine Curing Agents: In aromatic polyamines, the amino group is directly connected to the aromatic ring. Compared to aliphatic polyamines, they are less basic, and the reaction rate is significantly reduced due to the steric hindrance of the aromatic ring. Heating is often required for further curing. However, the cured products are superior to those of aliphatic amine systems in terms of heat resistance and chemical resistance. Aromatic polyamines must be modified, such as by forming adducts, or by adding catalysts such as phenol, salicylic acid, or benzyl alcohol, to create effective curing agents. These modified curing agents can cure at low temperatures, have low heat generation after mixing, excellent corrosion resistance, and resistance to acids and hot water. They are widely used in factory floor coatings, offering resistance to spills and abrasion.
Other Amine Curing Agents:
① Dicyandiamide: It has long been used as a latent curing agent in powder coatings, adhesives, and other fields. Dicyandiamide can cure epoxy resins within 30 minutes at 145–165°C, but it is relatively stable at room temperature. When solid dicyandiamide is thoroughly pulverized and dispersed in liquid resin, its storage stability can reach 6 months. When pulverized together with solid resin to make powder coatings, it exhibits good storage stability.
② Oxalic acid dihydrazide: The complex formed with epoxy resin is stable at room temperature and only slowly dissolves and undergoes a curing reaction upon heating. Accelerators such as tertiary amines and imidazoles can also be added to speed up the curing reaction.
③ Ketimine compounds: These are latent curing agents. When a paint film made by mixing them with epoxy resin is exposed to air, the ketimine compounds absorb moisture from the air to produce polyamines, thus causing the paint film to cure rapidly.
④ Mannich addition polyamines: The Mannich reaction is a condensation reaction of phenol, formaldehyde, and polyamine. Its curing characteristic is that it can cure even in low-temperature and humid environments. It is often used for epoxy resin paints that require rapid curing during cold seasons.
Tertiary amine and imidazole curing agents
Tertiary Amine Curing Agents:
Tertiary amines are Lewis bases. Their molecules do not contain active hydrogen atoms, but the nitrogen atom still has a lone pair of electrons, which can nucleophilically attack the epoxy group, catalyzing the self-curing of the epoxy resin. This is an anionic catalytic reaction. Tertiary amine curing agents have the disadvantages of varying curing agent dosage, curing speed, and curing product performance, as well as generating a large amount of heat during curing, making them unsuitable for large-scale casting. The most typical tertiary amine curing agent is DMP-30 (or K-54). The amino group in this compound’s molecule does not have active hydrogen atoms and cannot combine with the epoxy group, but it can promote the cross-linking of polyamides, thiols, etc., with the epoxy group.
Imidazole Curing Agents:
Imidazole curing agents are a new type of curing agent that can cure epoxy resins at lower temperatures, resulting in cured products with excellent heat resistance and mechanical properties. Imidazole curing agents are mainly 1-, 2-, or 4-substituted imidazole derivatives. The properties of imidazole curing agents vary depending on their structure. Generally, the stronger the basicity of the imidazole curing agent, the lower the curing temperature. The imidazole ring contains two nitrogen atoms. The lone pair of electrons on the 1-position nitrogen atom participates in the formation of the aromatic π-bond within the ring, while the lone pair of electrons on the 3-position nitrogen atom does not. Therefore, the basicity of the 3-position nitrogen atom is stronger than that of the 1-position nitrogen atom, and the 3-position nitrogen atom mainly plays the catalytic role. The substituent on the 1-position nitrogen atom has a significant impact on the reactivity of the imidazole curing agent. When the substituent is large, the lone pair of electrons on the 1-position nitrogen atom cannot participate in the formation of the aromatic π-bond within the ring, and in this case, the 1-position nitrogen atom acts similarly to a tertiary amine.
Boron-amine complexes and amine-containing borate ester curing agents
Boron Trifluoride-Amine Complex Curing Agents:
The boron atom in boron trifluoride molecules is electron-deficient and readily combines with electron-rich substances. Therefore, boron trifluoride is a Lewis acid and can combine with the epoxy group in epoxy resins, catalyzing the cationic polymerization of epoxy resins. Boron trifluoride is highly reactive; when mixed with glycidyl ester-type epoxy resins at room temperature, it cures quickly and releases a large amount of heat. Furthermore, boron trifluoride is easily deliquescent in air and is irritating, so it is generally not used alone as an epoxy resin curing agent. Typically, boron trifluoride is combined with a Lewis base to form a complex, thereby reducing its reactivity. The Lewis bases used are mainly monoethylamine, but also include n-butylamine, benzylamine, and dimethylaniline. Boron trifluoride-amine complexes are stable when mixed with epoxy resins at room temperature, but at high temperatures, the complex decomposes to produce boron trifluoride and amine, which then quickly react with the epoxy resin to initiate curing. The most representative boron trifluoride-amine complex curing agent is the boron trifluoride monoethylamine complex, whose structural formula is shown below. It is stable when mixed with epoxy resin at room temperature, but when heated to above 100°C, the complex decomposes into boron trifluoride and ethylamine, which then initiates the curing of the epoxy resin. The reactivity of boron trifluoride-amine complexes mainly depends on the basicity of the amine. For weakly basic amines such as aniline and monoethylamine, the reaction initiation temperature of their complexes is low, while for strongly basic amines such as piperidine and triethylamine, the reaction initiation temperature of their complexes is higher.
Amine-containing Borate Ester Curing Agents:
This type of curing agent is a class of amine-containing cyclic borate ester compounds successfully developed in my country in the 1970s. The advantages of these curing agents include high boiling point, low volatility, low viscosity, low skin irritation, good compatibility with epoxy resins, and ease of handling. Mixtures with epoxy resins maintain relatively stable viscosity for 4-6 months at room temperature, resulting in a long shelf life and good cured product performance. The disadvantage is that they are easily hygroscopic and deliquescent in air; therefore, they must be stored in sealed containers to prevent moisture absorption. For example, 901 curing agent, when cured with epoxy resin at 150°C for 5 hours, has a working life of two weeks at room temperature. When used in a polyamide-epoxy system, it does not gel after 14 months of storage at room temperature, but cures in 30-60 seconds when baked at 190-260°C. The cured product exhibits excellent mechanical properties.
Anhydride Curing Agents:
The advantages of anhydride curing agents include low skin irritation, a long working life when mixed with epoxy resins at room temperature, and excellent cured product performance, especially superior dielectric properties compared to amine curing agents. Therefore, anhydride curing agents are mainly used in electrical insulation applications. Their disadvantages include high curing temperatures, often requiring heating to above 80°C for the curing reaction to occur, resulting in longer molding cycles compared to other curing agents. The types of modifications are also limited, and they are often used as eutectic mixtures. In the absence of accelerators, anhydride curing agents react with the hydroxyl groups in the epoxy resin, producing a monoester containing a carboxyl group, which then initiates the epoxy resin curing. The curing reaction rate is related to the hydroxyl group concentration in the epoxy resin; epoxy resins with very low hydroxyl group concentrations have very slow curing reaction rates, while those with high hydroxyl group concentrations have faster curing reaction rates. The amount of anhydride curing agent used is generally 0.85 times the molar amount of epoxy groups. It can be seen that the curing reaction rate depends on the concentration of the tertiary amine; the higher the tertiary amine concentration, the faster the curing reaction rate. Each anhydride molecule corresponds to one epoxy group, and the amount of anhydride used is equal to the stoichiometric amount of epoxy groups. Tertiary amines are the most commonly used accelerators for anhydride-cured epoxy resins. Due to their high reactivity, tertiary amines are usually used in the form of carboxylic acid salts. Commonly used tertiary amine accelerators include triethylamine, triethanolamine, benzyldimethylamine, dimethylaminomethylphenol, tris(dimethylaminomethyl)phenol, and 2-ethyl-4-methylimidazole. In addition to tertiary amines, quaternary ammonium salts and organometallic compounds such as zinc naphthenate and zinc hexanoate can also be used as accelerators for anhydride/epoxy resin curing reactions. There are many types of anhydride curing agents, which can be classified into linear aliphatic anhydrides, aromatic anhydrides, and alicyclic anhydrides according to their chemical structure. They can also be classified into monofunctional anhydrides and bifunctional anhydrides according to the number of anhydride functional groups. They can also be classified according to whether they contain free carboxyl groups in the molecule. Different types of anhydride curing agents have different properties and uses. Maleic anhydride can also be used as a curing agent for epoxy resins; for 100g of bisphenol A epoxy resin, the amount of maleic anhydride is 30-40g. Maleic anhydride is strongly acidic, and its curing rate for epoxy resins is relatively fast. Maleic anhydride can also undergo addition reactions with various conjugated dienes to produce a variety of important liquid anhydrides. For example, maleic anhydride and butadiene can synthesize 70-anhydride. 70-anhydride is a liquid anhydride with low toxicity and low volatility. Its dosage is 80% of the epoxy resin amount, and the curing conditions are 150℃ for 4 hours or 180℃ for 2 hours. Tung oil-modified maleic anhydride can be used to produce liquid tung oil anhydride (308 anhydride). For every 100g of bisphenol A resin, 200g of this anhydride is used. The curing conditions are 100–120℃ for 4 hours. The cured product is soft and has good elongation, but a low heat distortion temperature. 647 anhydride is a low-melting point mixed anhydride, composed of the adduct of cyclopentadiene and maleic anhydride, and some unreacted maleic anhydride. Its melting point is below 40℃. The actual amount used is 80%–90% of the calculated value. The curing conditions are 150–160℃ for 4 hours, and the heat distortion temperature of the cured product is 150℃.
Synthetic resin curing agents
Many synthetic resins used in coatings contain phenolic hydroxyl groups, alcoholic hydroxyl groups, or other active hydrogen atoms. At high temperatures (150–200°C), these can cure epoxy resins, resulting in cross-linked paint films with excellent properties. These synthetic resin-based curing agents mainly include phenolic resin curing agents, polyester resin curing agents, amino resin curing agents, and liquid polyurethane curing agents. By changing the type and ratio of the resins, coatings with different properties can be obtained.
Phenolic Resin Curing Agents:
Phenolic resins contain a large number of phenolic hydroxyl groups, which can cure epoxy resins under heating conditions, forming a highly cross-linked, high-performance phenolic-epoxy resin film. The film retains both the excellent adhesion of epoxy resins and the heat resistance of phenolic resins, thus exhibiting excellent acid and alkali resistance, solvent resistance, and heat resistance. However, the film has a dark color and cannot be used for light-colored paints. It is mainly used for coating the inner walls of cans, packaging drums, storage tanks, and pipes, as well as chemical equipment and electromagnetic wires.
Amino Resin Curing Agents:
Amino resins mainly refer to urea-formaldehyde resins and melamine-formaldehyde resins. Both urea-formaldehyde and melamine-formaldehyde resins contain hydroxyl and amino groups in their molecules, which can react with epoxy groups to cure epoxy resins, resulting in paint films with good chemical resistance and flexibility. The paint films are light in color and have high gloss. They are suitable for coating medical devices, instruments, and for topcoating metal or plastic surfaces. Butanol-etherified urea-formaldehyde resin has good compatibility with epoxy resin, and butanol-etherified melamine-formaldehyde resin is also miscible with epoxy resin. The best performance of the paint film is achieved when the mass ratio of epoxy resin to amino resin is 70:30. When the proportion of epoxy resin increases, the flexibility and adhesion of the paint film improve. When the proportion of amino resin increases, the hardness and solvent resistance of the paint film improve.
Liquid polyurethane curing agent:
Polyurethane molecules contain both amino groups and isocyanate groups, which can react with the epoxy groups or hydroxyl groups in epoxy resins, thus curing the epoxy resin. The resulting paint film exhibits superior water resistance, solvent resistance, chemical resistance, and flexibility, and can be used for coating water-resistant equipment or chemical equipment.
Polysulfide Rubber Curing Agents:
Polysulfide rubber curing agents mainly include liquid polysulfide rubber and polysulfide compounds.
Liquid Polysulfide Rubber:
Liquid polysulfide rubber is a viscous liquid with a relative molecular weight typically ranging from 800 to 3000. After vulcanization, liquid polysulfide rubber exhibits excellent elasticity and adhesion, and is resistant to various oils and chemical media, making it a versatile sealing material. Liquid polysulfide rubber molecules contain thiol groups (-SH) at their ends, which can react with epoxy groups, thus curing epoxy resins. Without an accelerator, the reaction is extremely slow. When a Lewis base is added as an accelerator, the reaction can proceed at low temperatures of 0–20°C. At room temperature, it has a working time of only 2–10 minutes, but complete curing takes about a week. Higher temperatures accelerate the curing speed and lead to a more complete reaction.
Polysulfide Compounds:
This polysulfide compound is a low-molecular-weight oligomer with thiol groups at its molecular ends. Unlike liquid polysulfide rubber, it cannot cure epoxy resins at low temperatures even with a Lewis base as an accelerator. However, when used in combination with ordinary tertiary amines or polyamine curing agents, polysulfide compounds can cure epoxy resins at room temperature.
Matters related to epoxy resin curing
The three stages of epoxy resin curing: Liquid – Working Time: The resin/hardener mixture is still liquid and suitable for application. Gel – Gelling Stage: The mixture begins to enter the curing phase (also known as the maturation stage), at which point it begins to gel or “transition” into a soft gel. At this point, only partial curing has occurred, and newly applied epoxy resin can still chemically bond with it; therefore, the untreated surface can still be bonded or reacted with. Solid – Final Curing: The epoxy mixture becomes a solid, at which point it can be sanded and shaped. Allow it to continue curing at room temperature for several days.
Curing Temperature: Low-temperature curing agents have curing temperatures below room temperature: modified amines, polythiols, and polyisocyanates. Room-temperature curing agents have curing temperatures of room temperature to 50°C: aliphatic polyamines, alicyclic polyamines; low-molecular-weight polyamides and modified aromatic amines. Medium-temperature curing agents are 50–100°C: some alicyclic polyamines, tertiary amines, imidazoles, and boron trifluoride complexes. High-temperature curing agents have curing temperatures above 100°C: aromatic polyamines, anhydrides, novolac phenolic resins, amino resins, dicyandiamide, and acyl hydrazides.
Cured product characteristics: For addition polymerization curing agents, curing temperature and heat resistance increase in the following order: aliphatic polyamines < alicyclic polyamines < aromatic polyamines ≈ phenolic resins < anhydrides, while the heat resistance of catalytic addition polymerization curing agents is generally at the level of aromatic polyamines. Color: (Excellent) Alicyclic → Aliphatic → Amide → Aromatic amine (Poor) Maturity: (Low) Alicyclic → Aliphatic → Aromatic → Amide (High) Pot life: (Long) Aromatic → Amide → Alicyclic → Aliphatic (Short) Curing speed: (Fast) Aliphatic → Alicyclic → Amide → Aromatic (Slow) Irritancy: (Strong) Aliphatic → Aromatic → Alicyclic → Amide (Weak) Gloss: (Excellent) Aromatic → Alicyclic → Polyamide-aliphatic amine (Poor) Flexibility: (Soft) Polyamide → Aliphatic → Alicyclic → Aromatic (Rigid) Adhesion: (Excellent) Polyamide → Alicyclic → Aliphatic → Aromatic (Good) Acid resistance: (Excellent) Aromatic → Alicyclic → Aliphatic → Polyamide (Poor) Water resistance: (Excellent) Polyamide → Aliphatic amine → Alicyclic amine → Aromatic amine (Good)
