The curing temperature and heat resistance vary greatly among the various hardeners. Generally, curing adhesives with high curing temperature can obtain epoxy resin cured products with excellent heat resistance. For curing agents with polymerizers, the heat resistance of epoxy resin cures is increased in the following order:
Aliphatic polyamines "alicyclic polyamines, aromatic polyanhydrides, phenolic anhydrides.
The heat resistance of catalytic addition polymerization curing agents is generally at the level of aromatic polyamines. The heat resistance of anionic polymeric (tertiary amine and imidazole compounds) and cationic polymeric (BF3 complex) curing agents is basically the same, although the reaction mechanism is different from that of bisphenol A epoxy resins, but in the end they all form a three-dimensional network structure bonded by ether bonds.
Because the curing reaction is a chemical reaction, the curing temperature increases, the reaction rate is accelerated, and the gelation time is shortened. The logarithm of gel time decreases in a straight line as the heating temperature increases. However, it is worth noting that the curing temperature is too high, and due to the uneven heating of the whole curing system, the cross-linking density distribution of the epoxy resin cured product is often uneven, so that the performance of the epoxy resin cured product is reduced. Therefore, there is an upper limit of curing temperature, and it is necessary to choose a suitable curing temperature that is favorable to both the curing rate and the properties of the epoxy cure. According to the curing temperature, the curing agent can be divided into four types:
Low-temperature curing agent that cures at room temperature;
Room temperature curing agent curing at room temperature to 50 °C;
50 100 medium temperature curing agent;
More than 100 high-temperature hardeners.
There are very few curing agents that belong to the low-temperature curing type, and there are only two curing agents, polyisocyanate and polymercaptan. There are many types of room temperature curing agents, such as aliphatic polyamines, alicyclic polyamines, low molecular weight polyamides and modified aromatic amines. Medium-temperature curing agents include alicyclic polyamines, tertiary amines, imidazoles and boron trifluoride complexes. High-temperature curing agents include aromatic amines, anhydrides, alpha phenolic resins, amino resins, dicyandiamide and hydrazides.
For high-temperature curing systems, the curing process is divided into two stages, initially curing at a lower temperature, and postcuring with high-temperature heating when the gel state or a slightly higher state than the gel state is reached (called precure).
Structure and properties of the curing agent
As mentioned above, the curing temperature varies greatly depending on the type of curing agent, so the heat resistance of epoxy resin cured products can vary greatly. Even if the curing agent with the same functional group in the same class has different characteristics due to its different chemical structure, the cured product of the epoxy resin obtained is also different. Taking a typical polyamine curing agent as an example, although the functional groups are the same, the properties of the curing agent and the properties of the epoxy resin cured product are different due to the different chemical structures.
The properties of polyamine curing agents are listed in the following order:
Hue (excellent) alicyclic aliphatic amide aromatic amine (inferior).
Viscosity (low) alicyclic aliphatic aromatic amide (high).
Trial period (long) aromatic amide alicyclic aliphatic (short).
Curing (fast) aliphatic alicyclic amide aromatic (slow).
Irritating (strong) aliphatic aromatic alicyclic amide (weak).
Chemical structure and properties of polyamine curing agents.
Glossy (excellent) aromatic, alicyclic polyamide, aliphatic (inferior).
Softness (soft) polyamide aliphatic alicyclic aromatic (rigid).
Adhesion (excellent) polyamide alicyclic aliphatic aliphatic aromatic (good).
Acid resistance (excellent), aromatic, aliphatic, aliphatic, polyamide (inferior).
Water resistance (excellent) polyamide, aliphatic, alicyclic, aromatic (good).
Physical properties of polyamine-cured DGEBA resins.