Epoxy resin requires a suitable chemical substance to form a three-dimensional network with different properties, and this suitable chemical substance is generally referred to as an epoxy resin curing agent. There are many types of epoxy resin curing agents, and the classification methods are also different. It can be roughly divided into low temperature curing agent, normal temperature curing agent and high temperature curing agent.
Epoxy resin curing agent
1. Structure and characteristics of curing agent
As mentioned above, the curing temperature of the curing agent is strongly related to the heat resistance of the cured product. Similarly, in the same type of curing agent, although they have the same functional group, due to different chemical structures, their properties and cured product characteristics are also different. Therefore, it is very important for the selection of curing agents to fully understand the properties and characteristics of polyamine curing agents with the same functional groups and different chemical structures.
In terms of hue, the alicyclic group is light and basically transparent, while the aliphatic and aromatic groups have a considerable degree of coloration. Viscosity is also very different, alicyclics are only a few Pa·s, while polyamides are very viscous, up to several Pa·s, and aromatic amines are mostly solid. The length of the pot life is exactly the opposite of the curability, the aliphatic reactivity is high, and the alicyclic, amide, and aromatic are sequentially reduced.
Hue: (excellent) cycloaliphatic→aliphatic→amide→aromatic amine (inferior)
Maturity: (low) alicyclic→aliphatic→aromatic→amide (high)
Applicable period: (long) aromatic→amide→alicyclic→aliphatic (short)
Cureability: (fast) aliphatic→alicyclic→amide→aromatic (slow)
Irritation: (strong) aliphatic→aromatic→alicyclic→amide (weak)
In addition, in terms of gloss, flexibility, adhesion, acid resistance, and water resistance, there is also a certain regularity.
Gloss: (excellent) aromatic→alicyclic→polyamide-fatty amine (inferior)
Softness: (soft) polyamide→aliphatic→alicyclic→aromatic (just)
Adhesiveness: (Excellent) polyamide→alicyclic→aliphatic→aromatic (good)
Acid resistance: (excellent) aromatic → alicyclic → aliphatic → polyamide (inferior)
Water resistance: (Excellent) polyamide→fatty amine→alicyclic amine→aromatic amine (good)
For gloss, aromatics are good and aliphatics are poor. This property is affected by the curing temperature. As the temperature increases, the gloss becomes better. As for flexibility, polyamides with long distances between functional groups are better, while aromatic amines with high crosslink density are poor. Heat resistance is the opposite of flexibility, while adhesion is consistent with flexibility. Chemical resistance (acid resistance) is affected by the chemical structure, aromatic is relatively good, fatty amines and polyamides are susceptible to chemical corrosion. Water resistance is dominated by the mass concentration of functional groups. Polyamides with a low mass concentration of functional groups and high hydrophobicity are more resistant to water, while aromatics with a high mass concentration of functional groups are worse.
2. Various epoxy resin curing agents with different uses
curing agent can be divided into room temperature curing agent and heating curing agent according to the application. As mentioned above, epoxy resins generally have good performance when cured at high temperature, but the coatings and adhesives used in civil construction require heating at room temperature due to the difficulty of heating; therefore, most of them use fatty amines, alicyclics, and polyamides. In particular, paints and adhesives used in winter have to be used in combination with polyisocyanate, or use polyalcohols with a foul odor.
As for the medium-temperature curing agent and the high-temperature curing agent, it should be selected based on the heat resistance of the object and the heat resistance, adhesiveness and chemical resistance of the cured product. The choice focuses on polyamines and anhydrides. Since the cured anhydride has excellent electrical properties, it is widely used in electronics and electrical appliances.
Excellent aliphatic polyamine cured product has excellent adhesion, alkali resistance and water resistance. Aromatic polyamines are also excellent in chemical resistance. Since the nitrogen element of the amino group forms a hydrogen bond with the metal, it has an excellent rust prevention effect. The higher the amine mass concentration, the better the anti-rust effect. The anhydride curing agent forms an ester bond with the epoxy resin, shows high resistance to organic acids and inorganic acids, and the electrical properties generally exceed polyamines.
Third, the curing mechanism of anhydride curing agent
Anhydride curing epoxy resin requires a small amount of alcohol, water, free acid and other accelerators in the resin/anhydride system, which can be cured slowly after heating. Therefore, the acid anhydride does not directly react with the epoxy group, and the ring of the acid anhydride must be opened.
1. The effect of active hydrogen on anhydride ring opening. The bisphenol A epoxy contains hydroxyl groups, which can open the acid anhydride. A hydroxyl group produces a carboxyl group, and a polyhydric alcohol can link two anhydride molecules to play a cross-linking role. Adding hydroxyl-containing compounds such as ethylene glycol, glycerin, hydroxyl-containing low-molecular-weight polyether, etc. can accelerate the ring-opening reaction. Water can cause the acid anhydride to produce two carboxyl groups, so humidity has an effect on the curing of the acid anhydride.
Esterification reaction: This is the main reaction of acid anhydride curing epoxy resin. The addition of carboxyl group and epoxy group produces ester group. The carboxyl group produced by the esterification reaction further opens the anhydride ring and reacts with the epoxy group to form a three-dimensional structure; at high temperatures, some carboxyl groups can catalyze the ring opening of the epoxy group to form a structure dominated by ether bonds.
2. The effect of tertiary amine (tertiary amine) on the ring opening of acid anhydride. The tertiary amine forms an ion pair with the acid anhydride, the epoxy group is inserted into this ion pair, the carboxy anion opens the epoxy group, generates an ester bond, and generates a new anion. For example, 2-ethyl-4 methylimidazole and 2,4,6-(N,N-dimethylaminomethyl)-phenol, namely K-54# (foreign name DMP-30#).