Types and properties of epoxy resin curing agent

Epoxy resin is usually used as the matrix of adhesives, coatings and composite materials, and is widely used in construction, machinery, electronic and electrical, aerospace and other fields
.
A complete concept of epoxy resin composition consists of four components (see Figure 1)
.
However, in actual application, it is not necessary to have all four components, but the resin component must include a curing agent, which shows the importance of the curing agent.
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Figure 1 The components of epoxy resin composition and their functions

The reason why epoxy resin can be widely used is the result of the varied combination of these ingredients
.

Types and properties of epoxy resin curing agent

1.
The definition of curing agent

The epoxy resin itself is a thermoplastic linear structure and cannot be used directly.
The second component must be added to the resin, and under certain temperature (or humidity) conditions, it can be added to the epoxy group of the epoxy resin.

Curing agent, also known as hardene agent, is an indispensable curing agent for thermosetting resins.
For epoxy resins, there are many varieties, and there are more types of curing agents.

2.
Types and properties of curing agent

There are many types of curing agents, and several commonly used curing agents are now classified as follows;

1.
Amine curing agent:

⑴ Polyamides: Polyamides used as curing agents for epoxy resins are made by reacting dimerized, trimerized vegetable oleic acid or unsaturated fatty acids with polyamine amides
.
Due to the long fatty acid carbon chain and amino group in the structure, the cured product can have high elasticity, adhesion and water resistance.
It also has good workability, a wide proportion of ingredients, low toxicity, and basically no volatiles.
, Can be constructed on wet metal and concrete surfaces
.
But its disadvantage is that the heat resistance is relatively low, the heat distortion temperature is only about 50 ℃; less than 15 ℃ curing is not complete, the physical and mechanical properties of the cured product will be reduced, so it is necessary to add an accelerator to adjust its curing speed.
However, an excessive amount will increase the brittleness of the cured product; the resistance to gasoline and hydrocarbon solvents is poor
.

⑵ Aliphatic amines: the amount of aliphatic amine curing agent in various curing agents is second only to polyamide
.

Aliphatic amine curing agents are often used in adhesives, sealants, small castings, laminates, and room temperature curing coatings that cannot be heated (such as large parts) or not allowed to be heated (heat sensitive parts)
.

⑶ Aromatic amines: The molecular structure of aromatic amine curing agents contains a stable benzene ring structure, and the amine group is directly connected to the benzene ring
.

The cured product has better heat resistance, chemical resistance, electrical and mechanical properties
.

⑷ Alicyclic amines: Alicyclic amines are amine compounds containing alicyclic (cyclohexyl, heterooxy, and nitrogen atom six-membered ring) in the molecular structure
.

⑸ Polyether amines: Polyether amines generally contain a primary amine group connected to one end of the polyether main chain, and the main chain generally has ethylene oxide (EO), propylene oxide (PO) or EO/PO mixed structure, so The name is "polyetheramine"
.

Polyetheramine cross-linked products can enhance the elasticity, toughness, impact resistance and flexibility of the cured product
.
Polyetheramine's low viscosity, low color and long operating time are very suitable for the production and production of epoxy jewelry adhesives
.

⑹ Imidazoles: imidazole curing agents are compounds containing imidazole structure in the molecular structure
.

Compared with other curing agents, the amount used is small, and the epoxy resin can be cured in a short time at medium temperature (80～120℃).
The heat distortion temperature of the cured product is high compared with aliphatic amines and aromatic amines.

The disadvantage of imidazole compounds is that they have a certain degree of volatility and hygroscopicity; many imidazole compounds are crystals with high melting points, which are difficult to mix with liquid epoxy resins, which brings inconvenience to the operation process
.

2.
Acid anhydride curing agent

⑴ Aromatic acid anhydride: Aromatic acid anhydride contains benzene ring in its molecular structure, and the cured product has good heat resistance, high heat distortion temperature and excellent electrical properties
.
Because it is solid, it has a higher melting point
.

⑵ Aliphatic anhydride: Aliphatic anhydride is prepared by the interaction of aliphatic dibasic acid and acetic anhydride
.
Since the molecular structure is an aliphatic long chain, it can impart toughness and thermal shock resistance to the cured resin
.
Such curing agents can be used alone or mixed with other acid anhydrides as curing agents for powder coatings and casting resins
.

⑶ Alicyclic anhydrides: alicyclic anhydrides and aromatic anhydrides are different, there is no benzene ring in the molecular structure, so the weather resistance of this type of anhydride is better than that of aromatic anhydrides
.

① Methyltetrahydrophthalic anhydride: Methyltetrahydrophthalic anhydride is a liquid with low volatility, low toxicity, and low viscosity.
It can be miscible with epoxy resin at room temperature, and its cured epoxy resin has electrical insulation properties , Mechanical strength, heat resistance and other comprehensive performance is relatively good, the price is relatively cheap, so it has a wide range of uses, mainly used for the impregnation of generators, locomotive motor coils, insulators, insulating bushings, transformers, transformers, televisions The potting of power transformers makes methyltetrahydrophthalic anhydride the most versatile new liquid acid anhydride curing agent
.

②.
Methylhexahydrophthalic anhydride: Methylhexahydrophthalic anhydride is the product of hydrogenation of methyltetrahydrophthalic anhydride
.
Compared with A-4, A-6 has the following characteristics:

a.
The molecular structure does not contain double bonds and is colorless and transparent, so it can be used to cast colorless and transparent cured products to produce light-emitting diodes, lenses, laser disks and other products
.

b.
Due to the relatively non-polar molecular structure, the cured product has good moisture resistance
.

c.
The heat distortion temperature of the cured product is relatively high, and the curing time required is relatively short, which is conducive to the dipping and molding of large motors
.

d.
Since there is no double bond in the molecular structure, the cured epoxy resin has better weather resistance
.

3.
Latent curing agent

Latent curing agents refer to compounds that have a long pot life in epoxy resins at room temperature and can quickly undergo curing reactions after heating or ultraviolet radiation
.
They are classified according to external energy .
Latent curing agent is the key raw material for preparing one-component epoxy resin adhesives, coatings, and casting materials
.

Dicyandiamide is the earliest epoxy resin latent curing agent used in industry.
It is a white crystalline powder with a molecular weight of 84, a melting point of 207 to 209°C, and a relative density of 1.
40.
It is soluble in water and ethanol
.
Due to the effect of the conjugated double bond and the cyano group in the molecule, the alkalinity of the alkyl-substituted guanidine is greatly reduced, which makes it and the epoxy resin mixture have considerable stability at room temperature; due to the high melting point, the curing temperature is high.
The biggest disadvantage is that in order to reduce the curing temperature of dicyandiamide and increase its curing speed, various accelerators are often used in conjunction with it
.

The cured dicyandiamide epoxy resin has excellent adhesion and does not color.
Its dosage has an impact on the performance of the cured product.
Therefore, it is necessary to choose different dosages according to different uses; dicyandiamide has poor water resistance and is required for water resistance applications In the case, it is better to use a small amount of dicyandiamide
.

Third, the development trend of curing agent

The curing agent is a core material that exerts the value of epoxy resin.
The properties of the cured product depend on the performance of the curing agent.
Therefore, the research road to the curing agent is of far-reaching significance
.
From the study of curing agent to the present, combined with the current situation at home and abroad, the current curing agent is facing the following challenges and changes
.

1.
Develop a curing agent with high activity and excellent heat resistance, modify polyether amine, aliphatic amine or mix and compound to prepare a curing system with high activity and heat resistance
.

2.
The combination of toughness and strength
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Because traditional epoxy resins have poor performance after curing, especially low toughness and brittleness, which greatly affect their use, improving the performance of epoxy resins requires improving toughness
.

3.
The curing environment overcomes the volatility and toxicity of the amine curing agent, and uses physical or chemical methods to modify the amine to promote the development of the room temperature curing agent
.

4.
The adaptability and specificity of special environment
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It can meet special environments such as humid, low temperature underground environment or underwater for reservoir dam repair
.

5.
The matching of curing agent and curing technology
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Combining a variety of curing technologies (thermal curing, microwave curing, light curing) and choosing the right curing agent may be able to obtain a cured product with excellent comprehensive performance
.

6.
Heating-type latent curing agent has great potential, and we can continue to study dicyandiamide and its modified products, organic acid hydrazides, boron-amine complexes, imidazoles, microcapsules and other latent curing agents
.