Epoxy resins are commonly used as a matrix for materials such as adhesives, coatings and composites, and are widely used in construction, machinery, electrical and electronic, aerospace and other fields. A complete concept of epoxy resin constituents by four aspects of the composition. However, in practice, it is not necessary to have all four aspects of the components, but the resin composition must include the curing agent, which shows the importance of the curing agent. Epoxy resin components Epoxy resin Epoxy resin Main part，bisphenol A type and other types of epoxy resins Curing agent Reacts with epoxy resins to form three-dimensional network polymers Components for modification Plasticiser Gives epoxy components plasticity, but reduces its heat and chemical resistance Toughening agent Improved impact resistance without compromising other properties Filler Increase weight, improve curability, mechanical properties, such as calcium carbonate, mica, etc Flame retardant agent To make the epoxy Components with flame resistance, there are filler flame retardant and reactive flame retardant Components used to regulate fluidity Diluent Reduce components viscosity, including active and inactive diluents Thixotropic agent Imparts thixotropic properties to epoxy compositions, e.g. asbestos, silica micropowder Other components Pigments, solvents, defoamers, levelling agents, tackifiers, etc. The reason why epoxy resins are widely used is the result of the versatile co-ordination of these components. Especially the curing agent, once the epoxy resin is determined, the curing agent plays a decisive role in the processability of the epoxy resin composition and the final performance of the cured product. Classification of epoxy curing agents 1. Classification by acid-base properties Type Curing agent name acidity Organic anhydrides, boron trifluoride and its complexes alkali Aliphatic diamines, polyamines, aromatic polyamines, dicyanodiamines imidazoles, modified amines   2. Classification by reactivity and chemical structure Curing agent Apparent curing agent Addition reaction Polyamine Simple amine Straight chain fatty amine DETA，TETA，DEPA，TEPA Polyamides Polyamides with different amine values Aliphatic amine MDA，IPDA Aromatic amine m-XDA, DDM, m-PDA, DDA Modified amine Anhydride Monofunctional group PA, THPA, HHPA, MeTHPA, MeHHPA, MNA, DDSA, HET Bifunctional group PMDA, BTDA, TMEG, MCTC Carboxyl group TMA, PAPA Polyphenol PN Polythiol PM, PS Catalyst reaction Anionic polymers DMP-30, 2E4MZ Cationic Polymers BF3∙MEA Latent curing agent Dicyandiamide Organic acid hydrazide Ketimine microcapsules   3. Classification by curing temperature Curing Temp. Curing agent type Curing agent name 0-20°C Low temperature curing agent Polythiols, aliphatic polyamines or promoters, aromatic polyamines or promoters 20-40°C Normal temperature curing agent Polyamide, tertiary amine 60-100°C Medium temperature curing agent Dibasic aminopropylamine, imidazole, tertiary amine salts, aliphatic amines 100-150°C Medium and high temperature curing agent Anhydride or promoter, BF3-ammonium salt, dicyandiamide/promoter, imidazole derivatives, hydrazides 150°C+ High temperature curing agent Aromatic polyamines, polyphenols, acid anhydrides   4. Classification by different usage Curing agent Curing at room temperature Heavy Duty Anti-corrosion Coatings Adhesives for civil engineering and construction Civil Engineering Coatings FRP General Adhesives Alicyclic polyamines Denatured polyamines Straight chain aliphatic polyamines Polyamides, polythiols Heat curing Electrically insulating material Acid anhydride, imidazoles, BF3 complexes Laminated materials Dicyandiamide, aromatic polyamines, linear phenolic resins Coatings   Tank materials Amino resins, methyl phenolic resins Powder material Dicyandiamide, aromatic polyamines, acid anhydrides Moulded material Linear phenolic resins Adhesives Aromatic polyamines, anhydrides, imidazoles, BF3 amine complexes     Structure and properties of curing agents A comprehensive understanding of the properties and characteristics of polyamine curing agents with the same functional group but different chemical structures is very important for the selection of curing agents. The main characteristics (color, ripeness, duration of use, etc.) also show a certain regularity. Color: (good) alicyclic->aliphatic->amide->aromatic amine(bad) Maturity: (low) alicyclic->aliphatic->aromatic->amide(high) Application period: (Long) Aromatic->Amide->Alicyclic->Aliphatic (Short Curability: (Fast) Aliphatic->Alicyclic->Amide->Aromatic (Slow) Irritation: (Strong) Aliphatic->Aromatic->Alicyclic-Amide (Weak) Gloss: (Excellent) Aromatic->Alicyclic->Polyamide->Aliphatic amide (Poor) Flexibility: (Soft) Polyamide->Aliphatic->Alicyclic->Aromatic (Rigid) Adhesion: (Excellent) Polyamide->alicyclic->aliphatic->aromatic (Good) Acid resistance: (Excellent) Aromatic->Alicyclic->Aliphatic->Polyamide (Inferior) Water Resistance: (Excellent) Polyamide->Aliphatic Amine->Aliphatic Cyclic Amine->Aromatic Amine (Good)     Development trend of curing agent Curing agent as a core substance to play the value of epoxy resin, the nature of the cured product depends on the performance of the curing agent, so the road of research on the curing agent has far-reaching significance. From the research of curing agent to date, combined with the current situation at home and abroad, curing agent is currently facing some of the following challenges and changes. The development of high activity and excellent heat resistance curing agent. The use of modified polyether amine, aliphatic amine or mixed compound to prepare high activity and heat resistance curing system. Due to the traditional epoxy resin in the curing performance is poor, especially low toughness, brittle, greatly affecting its use, so improve the performance of epoxy resin needs to improve the toughness. Improve the curing environment, overcome the volatility and toxicity of amine curing agent, and promote the development of room temperature curing agent by modifying amine with physical or chemical method. Improve the adaptability and specialty of epoxy resin in special environments, to meet the special environments such as humid, underground low temperature environment or underwater of reservoir dam repair. Curing agent and curing technology matching, will be a variety of curing technology (heat curing, microwave curing, light curing) combined with the selection of the appropriate curing agent may be able to get a comprehensive performance of the curing product. Heating type latent curing agent has great potential, can continue to study the dicyandiamide and its modified products, organic acid hydrazide, boron - amine complex, imidazole, microcapsules and other latent curing agent.