What is epoxy resin?
Epoxy resin is a type of thermosetting resin that contains epoxy groups (epoxy rings, -CH-CH₂-O) in its molecules, and is a polymer material with excellent adhesive properties, chemical resistance, mechanical strength, and electrical insulation. Epoxy resins harden through a chemical reaction with a curing agent and are used in many fields.
Epoxy resins have the following properties:
- Excellent adhesion: Strong adhesion to many materials (metal, glass, plastic, etc.).
- Chemical resistance: High resistance to acids, alkalis and solvents.
- High mechanical strength: Improved rigidity and durability after curing.
- Electrical insulation: Excellent electrical properties allow use as an insulating material.
- Workability: Properties can be adjusted by selecting the hardener and filler.
Epoxy resin design
Selection of curing conditions
| Curing system | Composition | Features |
| Heat (anionic polymerization) | Epoxy resin + hardener + hardening accelerator | ・Low cost ・Wide range of designs |
| Heat (cationic polymerization) | Epoxy resin + thermal cationic polymerization initiator | ・Low temperature polymerization ・Fast curing ・Epoxy homo-polymerization ・One-liquid process possible |
| Light (cationic polymerization) | Epoxy resin + photo cationic polymerization initiator | ・Environmentally friendly (low energy) ・Fast curing ・Epoxy homo-polymerization ・One-liquid process possible |
Epoxy resin selection
- Bisphenol type epoxy resin
The most common epoxy resin. It is made from bisphenol A or bisphenol F and epichlorohydrin. It is used in adhesives, coatings, electrical insulation materials, etc.
・Bisphenol A type (high viscosity, high heat resistance)
・Bisphenol F type (low viscosity, excellent processability) - Novolac epoxy resin
Its structure is based on phenol novolac resin. Heat resistance and mechanical strength improve when it reacts with a hardener. It also has high chemical resistance, and is used in electrical insulating materials and paints for high-temperature applications that require heat resistance. - Allylcyclohexane type epoxy resin
Its unique ring structure gives it high weather resistance and transparency, and it is used in optical materials and special coatings. - Aliphatic Epoxy Resin
Resins based on allyl glycidyl ether and glycidyl ether. Excellent flexibility and transparency. Used for parts that require flexibility and for transparent coatings. - Cycloaliphatic epoxy resin
A resin based on cyclic alkanes. It has high UV stability and excellent resistance to yellowing. It is used in UV-curable coatings and electronic materials. - Modified epoxy resin
Epoxy resins are mixed with other polymers and chemicals to improve their properties and are used in applications where durability or specific properties are required.
・Silicone modification (improved flexibility)
・Polyurethane modification (improved impact resistance) - Water-based epoxy resin
A water-borne epoxy resin. It is environmentally friendly. It is used in water-based paints and adhesives.
Points to consider when selecting epoxy resin
・Heat resistance: Novolac type for use in high temperature environments.
・Transparency: Allylcyclohexane type or cycloaliphatic type for optical applications.
・Environmental friendliness: Consider water-based or UV-curing types.
・Flexibility: Aliphatic epoxy resins or modified epoxy resins.
It is important to select the appropriate type depending on the application and characteristics.
Selection of hardener
There are many different types of epoxy resin hardeners, each with different characteristics and uses. The main ones are listed below. The choice of epoxy resin and hardener depends on the purpose and required performance, so it is important to choose the right one for the application. For detailed product information, it is a good idea to refer to the manufacturer’s website, catalog, or technical documents.
- Amine-based hardener
It is highly reactive and hardens at relatively low temperatures such as room temperature. It is used in adhesives, coatings, electrical insulation materials, etc. However, it is hygroscopic and may take on a yellowish tinge.
・Polyethylene polyamine（PEPA）
・Triethylenetetramine（TETA） - Acid anhydride hardener
A commonly used hardener with excellent heat resistance, chemical resistance, and electrical insulation. Used in electronic components, insulators, and heat-resistant materials. Because of its slow curing speed, it is often used in cases where heating is required or in combination with an epoxy resin curing accelerator.
・Phthalic anhydride（PA）
・Methyl Nadic Anhydride（MNA） - Phenol novolac hardener
It has excellent strength and chemical resistance at high temperatures. It is used in high performance coatings, heat-resistant components, IC encapsulation materials, and laminates. However, it requires a high-temperature curing process. - Dicyandiamide (DICY) hardener
It is a hardener with very high thermal latency and has excellent mechanical and electrical properties. It is used in prepregs, which are materials for printed wiring boards, powder coatings, etc. - Cationic polymerization hardener (cationic polymerization initiator)
By using a thermal acid generator or photoacid generator, it can be cured with heat or ultraviolet (UV) light. Epoxy resin can be polymerized by itself. It is used for electronic materials that require fast curing and for heat-sensitive components.
Key points for selecting epoxy resin hardeners
・Usage environment: temperature, humidity, exposure to chemicals, etc.
・Cure speed: Do you need it to dry quickly or cure slowly?
・End product properties: mechanical strength, heat resistance, electrical insulation.
・Working conditions: Room temperature or heat curing.
・Cost and safety: Ease of use and price are also important factors.
Since hardeners can be irritating or toxic, please use appropriate safety equipment (gloves, masks, goggles, etc.) when working with them. It is also important to store them away from moisture and high temperatures to maintain their quality.
Selection of epoxy resin curing accelerator
Some types of curing agents have low reactivity with epoxy resins, so epoxy resin curing accelerators are used to accelerate the curing reaction of epoxy resins. Generally, 0.1 to 2.0 wt% is added to the epoxy resin. As shown below, different types of epoxy resin curing accelerators must be used depending on the application and purpose. Note that epoxy resin curing accelerators cannot be used to cure epoxy resins alone.
Also, if the curing agent is polyamine-based, the curing speed is already fast, so no curing accelerator is used.
- Tertiary amine type
・DMP-30：2,4,6-Tris(dimethylaminomethyl)phenol
・DBU：1,8-Diazabicyclo[5.4.0]-7-undecene
There are also salts of DBU with organic acids to provide thermal latency. - Imidazoles type
Imidazole is a curing accelerator that is often used especially in electronic materials and high-performance resins. Its characteristics include being effective at relatively low concentrations, providing excellent heat resistance and electrical properties, and easy control of the curing speed.
・ 2-Methylimidazole
・2-Ethyl-4-methylimidazole - Phosphorus type
It is commonly used as a phenolic hardener.
・TPP：Triphenylphosphine
Manufacturing method of epoxy resin
The curing temperature and time of epoxy resin are adjusted according to the type of curing agent and its application.
| Hardeners | Curing temperature | Main applications |
| Amine | 25～150°C | Adhesives, Coatings, Composites |
| Acid anhydride | 120～180°C | Electronic components, insulating materials |
| Phenol | 150～250°C | Heat-resistant parts, FRP (fiber reinforced plastic) |
| UV/electron beam curing | Low temperature (light irradiation) | Inks, coatings and 3D printing |
➡ Generally, it will harden completely after heating at 100 to 180°C for several hours.
Cationic Polymerization Reaction Mechanism
・Upon exposure to ultraviolet (UV) light, the photoinitiator decomposes and
produces an acid.
・The generated acid opens the epoxy group, initiating cationic polymerization.
・The cross-linked structure grows and hardening is completed in a short time.
Heat curing using a hardener
＜Reaction with amine hardeners (addition ring-opening reaction)＞
Amine-based hardeners undergo an addition reaction with epoxy groups to form a crosslinked structure.
This is the most common hardener, and produces hardened products with excellent heat resistance and mechanical strength.
An example of a reaction with diamines (polyamines) is shown below.
- Preparation of raw materials
・Epoxy resin: Bisphenol A type epoxy resin (DGEBA)
・Hardener: Ethylenediamine (EDA), diethyltriene (DETA) - Ring-opening addition reaction of epoxy groups and amine groups
The amine groups then react with other epoxies to crosslink. The reaction generally proceeds at room temperature and no accelerator is required.
＜Reaction with anhydride hardeners (ring-opening addition reaction)＞
Acid anhydride curing agents react with epoxy groups to form highly heat-resistant crosslinked structures.
An example of a reaction with phthalic anhydride (PA) is shown below.
- Preparation of raw materials
Epoxy resin: Bisphenol A epoxy resin (DGEBA)
Hardener: Phthalic anhydride (PA), trimellitic anhydride (TMA) - Acid anhydride reacts with epoxy group through ring-opening addition reaction
The cross-linking structure progresses, forming a hardened product with high heat resistance.
Thermal cationic polymerization
- Preparation of raw materials
・Monomer: Epoxy resins such as diglycidyl ether (DGEBA)
Bisphenol A epoxy resin (DGEBA)
Cycloaliphatic Epoxy
・Thermal initiator: Sulfonium salt
(e.g. Benzylmethyl p-hydroxyphenylsulfonium=Hexafluoroantimonate) - Cationic curing
・A thermal initiator (approximately 0.5 to 2.0 wt%) is mixed into the epoxy resin
・Heating (80-150°C) → Thermal initiator decomposes and generates cations
・Epoxy groups initiate cationic ring-opening polymerization
Completely hardens within a few seconds to a few minutes
(quick-drying, suitable for coating applications)
・Post-cure promotes further cross-linking
➡ Applications: High heat resistant resins (electrical insulation, aerospace materials), chemical resistant coatings
Photo cationic polymerization
- Preparation of raw materials
・Monomer: Epoxy resins such as diglycidyl ether (DGEBA)
Bisphenol A epoxy resin (DGEBA)
Cycloaliphatic Epoxy
・Photoinitiators: Sulfonium salts (e.g. triphenylsulfonium hexafluoroantimonate) - Cationic curing
・Photoinitiator (0.001 to 2.0 wt%) is mixed into epoxy resin
・Irradiate with ultraviolet (UV) light → Photoinitiator decomposes and generates cations
・Epoxy groups initiate cationic ring-opening polymerization
Completely hardens within a few seconds to a few minutes
(quick-drying, suitable for coating applications)
・Post-treatment (post-cure) promotes further crosslinking
➡ Applications: Insulation coating for electronic circuit boards, optical lens adhesives, resin for 3D printers