I. Overview of Curing Agents
(1) Definition of Curing Agents
Any substance that can react with the epoxy group and hydroxyl group of epoxy resin to crosslink and solidify the resin is called a curing agent, also called a hardener or a crosslinking agent. Since different curing agents will directly affect the process and physical and chemical properties of the product, it is very important to select epoxy resin curing agents according to the application, such as the high or low curing process temperature; the speed of curing rate; the hardness or softness of the required product performance; the temperature range of the use conditions; and the dryness and humidity of the use environment. Therefore, the curing agents used in different situations are different. In short, it is necessary to select the appropriate curing agent according to the actual situation in order to bring out the best performance of the epoxy resin system used.
(2) The important role of curing agent:
Epoxy resin is a kind of thermosetting polymer material with good adhesion, electrical insulation and chemical stability. It can be used as the resin matrix of adhesives, coatings and composite materials, and is widely used in the fields of construction, machinery, electronics and electrical, aerospace, etc. However, epoxy resin is a linear thermoplastic resin. It will not harden by itself and has no performance. Only when curing agent is added and reacted with curing agent, it will crosslink from linear structure to three-dimensional network or body structure to form insoluble and infusible material, and then it will have excellent performance and practical value. In addition, the performance of the cured product depends on the curing agent to a large extent. The structure and quality of the curing agent directly affect the application effect of epoxy resin. Therefore, the curing agent is an important component of epoxy resin after molding. Every time a new curing agent is developed, especially the addition curing agent combined with epoxy resin, it can solve a problem in a certain aspect, which is equivalent to developing a new epoxy resin or opening up a new use of epoxy resin. Therefore, curing agent plays an indispensable and even decisive role in the application of epoxy resin.
2. Classification of curing agents
Classification by chemical structure of curing agent：
a. Amine curing agents: aliphatic amines include polyamides, alicyclic amines, aromatic amines, amides, Mannich addition amines, and latent amines; tertiary amine and imidazole curing agents include tertiary amines and imidazoles; boron amine complexes and boric acid ester curing agents with amino groups include boron trifluoride-amine complexes.
b. Anhydride curing agents: straight-chain aliphatic anhydrides, aromatic anhydrides, and alicyclic anhydrides.
c. Synthetic resin curing agents: phenolic resin curing agents, polyester resin curing agents, amino resin curing agents, liquid polyurethane curing agents, etc.
d. Polysulfide rubber curing agents: liquid polysulfide rubber and polysulfide compounds.
3. Types and characteristics of curing agents
1. Amine curing agents
Amine curing agents include polyamine curing agents, tertiary amine and imidazole curing agents, boron amine and its boron amine complex curing agents.
(1) 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. However, the curing agents with greater demand are polyamide and aliphatic amine.
① Aliphatic polyamine curing agents Commonly used aliphatic polyamine curing agents include diethylenetriamine, triethylenetetramine, tetraethylenepentamine, hexamethylenediamine, meta-xylylenediamine, etc. They can cure epoxy resin at room temperature with fast curing rate and low viscosity. The curing effect of primary and secondary amines on epoxy resin is that the active hydrogen on the nitrogen atom opens the epoxy group, thereby causing cross-linking and curing. Aliphatic polyamines are more active and can crosslink and cure epoxy resins at room temperature; aromatic polyamines are less active, such as m-phenylenediamine, which needs to be cured at 150°C to be completely cured. Aliphatic polyamines are the earliest epoxy resin curing agents used. They can quickly cure bisphenol A epoxy resins at room temperature, but they are not very active for other epoxy resins except glycidyl epoxy resins.
② Polyamide polyamine curing agent is a modified polyamine, which is formed by condensation of vegetable oil fatty acids and polyamines. The product containing acylamino and amino groups has 3 active hydrogen atoms that can react with epoxy groups. It is not sensitive to environmental humidity and has good wettability to the substrate.
③ Alicyclic polyamine curing agents have light color, good color retention, low viscosity, but slow reaction, and often need to be used in combination with other curing agents, or add accelerators, or make adducts, or need to be heated for curing.
④ Aromatic polyamine curing agent. The amino group in aromatic polyamine is directly connected to the aromatic ring. Compared with aliphatic polyamine, the alkalinity is weak, and the reaction is affected by the steric hindrance of the aromatic ring. The curing rate is greatly reduced, and heating is often required for further curing. However, the cured product is better than the cured product of the aliphatic amine system in terms of heat resistance and chemical resistance. Aromatic polyamines must be modified to make adducts, etc., or add catalysts such as phenol, salicylic acid, benzyl alcohol, etc., to make a good curing agent that can be cured at low temperatures. Aromatic polyamine curing agents mainly include 4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenyl sulfone, metaphenylenediamine, etc.
⑤ Other amine curing agents
a. Dicyandiamide has long been used as a latent curing agent in powder coatings, adhesives and other fields. Dicyandiamide can cure epoxy resin within 30 minutes at 145-165℃, but it is relatively stable at room temperature. The solid dicyandiamide is fully crushed and dispersed in the liquid resin, and its storage stability can reach 6 months. It is crushed together with the solid resin to make a powder coating with good storage stability.
b. Mannich addition polyamine Mannich reaction is a condensation reaction of phenol, formaldehyde and polyamine. There are phenolic hydroxyl groups in the molecule, which can promote curing. Its curing characteristic is that it can cure even in low temperature and humid environment. It is often used for epoxy resins that need to be cured quickly in cold seasons.
(2) Tertiary amine and imidazole curing agents
① Tertiary amine curing agents Tertiary amines are Lewis bases. There are no active hydrogen atoms in their molecules, but there is still a pair of lone electrons on the nitrogen atom, which can carry out nucleophilic attack on the epoxy group and catalyze the self-ring-opening curing of epoxy resin. It is an anionic catalytic reaction. Tertiary amine curing agents have the disadvantages of large changes in curing agent dosage, curing rate, and curing product performance, and large heat release during curing. Therefore, they are not suitable for large-scale casting. Representative tertiary amine curing agents include triethanolamine, tetramethylguanidine, N,N′-dimethylpiperazine, triethylenediamine, and benzyldimethylamine.
② Imidazole curing agents Imidazole compounds can be cured at lower temperatures to obtain a cured product with excellent heat resistance and excellent mechanical properties. The reactivity of imidazole compounds varies according to their structure. Generally speaking, the stronger the alkalinity, the lower the curing temperature. Imidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, etc. are all highly active curing agents. They can cure epoxy resin in a short time at medium temperature. Therefore, the single-component system composed of them and epoxy resin has a short storage period.
(3) Boron trifluoride-amine complex curing agent
The boron atom in the boron trifluoride molecule lacks electrons and is easy to combine with electron-rich substances. Therefore, boron trifluoride is a Lewis acid and can combine with the epoxy in the epoxy resin to catalyze the cationic polymerization of the epoxy resin. Boron trifluoride is very active. After mixing with glycerol ester type epoxy resin at room temperature, it cures quickly and releases a large amount of heat. In addition, boron trifluoride is easy to deliquesce in the air and is irritating. Therefore, it is generally not used alone as a curing agent for epoxy resin. Boron trifluoride is usually combined with a Lewis base to form a complex to reduce its reaction activity. The Lewis base used is mainly monoethylamine, in addition to n-butylamine, benzylamine, dimethylaniline, etc. The advantages of this type of curing agent are high boiling point, low volatility, low viscosity, low skin irritation, good compatibility with epoxy resin, and easy operation. The viscosity of the mixture with epoxy resin does not change much after being kept at room temperature for 4 to 6 months, the storage period is long, and the performance of the cured product is good. The disadvantage is that it is easy to absorb water and deliquesce in the air, so it should be sealed during storage to prevent moisture absorption.
1 Advantages of anhydride curing agents
① Low volatility, low toxicity, and low irritation to the skin;
② Large amount of epoxy resin, low viscosity after mixing with epoxy resin, more fillers can be added for modification, which is conducive to reducing costs;
③ Long service life and easy operation. This type of curing agent has good storage stability, excellent performance of the cured product, good mechanical properties and heat resistance of the cured product, especially the dielectric properties are better than amine curing agents, so anhydride curing agents are mainly used in the field of electrical insulation. In the absence of accelerators, anhydride curing agents react with hydroxyl groups in epoxy resin to produce monoesters containing a carboxyl group, which in turn initiates the curing of epoxy resin. The curing reaction rate is related to the hydroxyl groups in the epoxy resin. The curing reaction rate of epoxy resin with a very low hydroxyl concentration is very slow, while the curing reaction rate of epoxy resin with a high hydroxyl concentration is fast. The amount of anhydride curing agent is generally 0.85 times the amount of epoxy substance.
(2) Disadvantages of anhydride curing agents
① Slow curing reaction and large shrinkage rate;
② Relatively high curing temperature is required;
③ Not easy to modify;
④ Easy to absorb moisture during storage to generate free acid, causing adverse effects (slow curing rate and reduced performance of the cured product);
⑤ The alkali and solvent resistance of the cured product are relatively poor. The curing temperature often needs to be heated to above 80°C for the curing reaction to proceed, so the molding cycle is longer than other curing agents, and the modification types are also limited, and it is often made into a eutectic mixture for use. The most commonly used accelerator for anhydride curing epoxy resin is tertiary amine. Due to its strong activity, tertiary amine is usually used in the form of carboxylic acid complex salts. Commonly used tertiary amine accelerators include triethylamine, triethanolamine, benzyldimethylamine, dimethylaminomethylphenol, tris(dimethylaminomethyl)phenol, 2-ethyl-4-methylimidazole, etc.
4. Selection of curing agent
There are several factors to consider in the selection of curing agent.
① Type and performance of curing agent. The type of curing agent has a great influence on the mechanical properties, heat resistance, water resistance, corrosion resistance, etc. of the cured product. The appropriate curing agent should be selected according to different uses and performance requirements.
② Compounding of several curing agents. The compounding of several curing agents can achieve a complementary effect.
③ Pay attention to the toxicity and environmental protection of the curing agent. The selected curing agent should be harmless to the human body and non-polluting to the environment. In order to meet the needs of the continuous development of science and technology and application fields, the research on modified curing agents and new curing agents has become the main direction of future research on curing agents. With the enhancement of people's environmental awareness, low-toxic, non-toxic and excellent curing agents will replace the old generation of curing agents and receive more attention and will be more widely used.
Our company's imidazole blocked curing agent can be cured within 30 minutes at 80 degrees Celsius and has good storage stability
