A wide variety of epoxy curing agents and curing technologies (thermal curing, microwave curing, UV curing, etc.) can be commonly used. Most of the UV curable epoxy resins are not suitable for thermal curing. Very few epoxy compounds, such as cycloaliphatic epoxy resins, are both UV curable and thermal curable. In the previous article, we introduced the UV curing application of cycloaliphatic epoxy resins. In this article, we will focus on another great advantage of cycloaliphatic epoxy resins —
Epoxy resin is generally used as a substrate of coatings, composites, adhesives and other materials. It is widely used in construction, electrical and electronic, aerospace and other fields. A complete epoxy resin system is generally composed of four parts, including epoxy resin, modifier, flowability regulator and some others (such as pigment, solvent, defoamer, leveling agent, tackifier, etc.). In fact it is not necessary to have all these four parts, but the resin part must include the very important curing agent.
The thermal curing agent for cycloaliphatic epoxy resins is mostly anhydride type. Commonly used anhydride type curing agent can be hexahydrophthalic anhydride (HHPA), methyl tetrahydrophthalic anhydride (MTHPA), chloroacetic anhydride, etc. It is less volatile, less toxic or less irritant to skin. It can be easily operated and used for a long time. Generally anhydride curing agent has low curing speed, but the cured material can provide good rigidity.
Cycloaliphatic epoxy resin/anhydride system has been widely used in various fields of the national economy due to its low viscosity, long service life, low curing heat, low shrinkage rate, strong heat resistance and excellent mechanical properties of curing products. The epoxy resin/anhydride system usually cures very slowly at low temperature and it can only be cured at high temperature. When electronic components are encapsulated, excessive temperatures may damage components, so catalysts are sometimes needed to reduce the curing temperature while speeding up the curing rate.
Anhydride-cured epoxy resin system possesses outstanding mechanical/physical properties and high-temperature stability.
The thermal curing reaction by anhydrides is divided into two types: with promoter and without promoter.
When in the absence of promoter, the hydroxyl group of the epoxy resin makes anhydride open ring to produce monoester and carboxylic acid.
The carboxylic acid is added to the epoxy group to produce diester and hydroxyl group.
The hydroxyl group obtained from esterification will react with epoxy group by etherification.
As such, the ring opening-esterification-etherification reaction continues until the epoxy glue is cross-linked and thermally cured.
When in the presence of Louis alkali (e.g., tertiary amine) promoter, tertiary amine will attack anhydride to produce carboxylate anion. The carboxylate anion reacts with epoxy group to produce alkoxide anion. Then the alkoxide anion reacts with another anhydride to produce another carboxylate anion. Addition polymerization is gradually done for curing of epoxy resins. When the promoter exists, anhydride is thermally cured and reacted to produce ester bond rather than ether bond. Please find the thermal curing mechanism of anhydride in the presence of the promoter as follows,
The tertiary amine attacks anhydride to produce carboxylate anion.
The carboxylate anion reacts with epoxy group to produce alkoxide anion.
The alkoxide anion reacts with another anhydride to produce another carboxylate anion.
When the promoter exists, the thermal curing speed of epoxy resin depends on the concentration of hydroxyl group in system. Hence, it is extremely effective for liquid epoxy resins to add with the promoter. The thermal curing reaction can be done at a temperature range from 120℃ to 150℃.
Cycloaliphatic epoxy resin (also called oxirane) is a low-molecular compound with two oxirane functional groups. It is not a polymer in itself, but it can react with the curing agent to create a high-performance, three-dimensional structured polymer.
Cycloaliphatic epoxy compounds have many advantages, including low viscosity, long service life, good weatherability, strong heat resistance and excellent mechanical properties. They are especially suitable for use in manufacturing of high-strength and heat-resistant composites by wet type laminating and winding technique.
The cycloaliphatic epoxy compound can also be used in the resin part of the plastic mold. It shows strong thermal resistance after curing. Due to good mechanical strength, low volume shrinkage ratio and high precision, it is suitable for use in precision molds and dies. When compared with metal molds, it shows advantages of easy processing, low price, light weight, convenient molding operation, etc.