Epoxy resin castable is a kind of multi-component composite system, which is composed of resin, curing agent, toughening agent, accelerator, filler, etc. Any kind of material and formula, craft, casting size and structure, molds, equipment and other factors will affect the performance and quality of the castings.
As one of the curing agents, methyltetrahydrophthalic anhydride, certainly plays an important role in the properties of castings.
Epoxy castable and viscosity should be as small as possible for ease of casting and removal of bubbles. But if the viscosity is too low, it is easy to cause the precipitation of the filler, resulting in uneven curing and cracking in the heat and cold exchange, so the viscosity of methyltetrahydrophthalic anhydride must be maintained stable.
2) Content and acid number
Epoxy resin castings are expected to be free of bubble gaps and cracks. The residential bubbles and air gaps are caused by air brought in by the addition of a large number of fillers, which requires the process to extend the vacuum defoaming time to solve the problem. Therefore, it is required that the reactivity of the castable is very low at the defoaming temperature and has a long application period. Generally speaking, epoxy resin castable requires a high content of methyltetrahydrophthalic anhydride and low impurities.
The curing reaction of acid anhydride and epoxy resin with high reactivity is bound to release a large amount of heat. But local overheating will result in the retention of thermal stress in the castings, which may cause cracks or air gaps at the interface with the workpiece. The activity of epoxy resin and the amount of residual alkali are also influencing factors, so the acid value should be controlled well when paying attention to the acid anhydride to avoid too high free acid.
During the curing reaction, the volume of the epoxy anhydride system begins to shrink, and when it reaches the gel, if there is no castable to supplement it adequately, there will be an air gap and internal stress. Pressure gels can effectively eliminate this problem, but the gelling time of castable must be stable enough and suitable enough.
3) Volatile matter
Another factor that causes bubbles, air gaps, cracks or shrinkage in epoxy castings is the low molecular volatile matter found in the filler moisture absorption, epoxy and curing agent. Therefore, in addition to ensuring the drying of the filler and the low volatile content of the epoxy resin, the low volatile content of methyltetrahydrophthalic anhydride should also be strictly controlled.
Methyltetrahydrophthalic anhydride has more than ten isomers due to its different CH3- position and C=C double bond position and the different three-dimensional bridge structure of benzene ring. Because of methyl group, there is a certain steric hindrance between the molecular structure of methyltetrahydrophthalic anhydride and the carbon atom of carboxy acid, and it is also the electron-donating group, so the induction effect exists. There two actions decrease the electrophilicity of the carboxyl carton atoms and reduce the activity of the anhydride group, thus giving a longer service life to the mixture of methyltetrahydrophthalic anhydride and epoxy resin. In the meantime, the double bond position in the methyltetrahydrophthalic anhydride molecule and carbonyl formed a stable conjugate, which attaches excellent moisture resistance to the methyltetrahydrophthalic anhydride. Therefore, the activity and moisture resistance of various isomers are very different.