Types of epoxy and hardener and three major applications

The wide variety of epoxy resin types is renowned for their excellent chemical resistance, heat resistance, and electrical insulation properties.

 

Matching them to various applications has become a focal point of interest for numerous developers.

 

The types of traditional epoxy resins

 

Bisphenol A Epoxy; DGEBA

 

Bisphenol A epoxy resin is a typical liquid epoxy resin formed by the condensation of bisphenol A and epichlorohydrin. It possesses excellent adhesion, chemical resistance, and heat resistance. Due to its exceptional properties, it finds wide applications in various fields, including:

 

1.Paints: Electrodeposition coatings, ambient cure coatings, transparent coatings, anti-corrosive coatings.

2.Electrical and electronic fields: Casting, impregnation, encapsulation, lamination, coatings for capacitors and resistors.

3.Civil engineering and construction: Cementitious concrete structures, waterproofing, anti-corrosion pavements, joint repair adhesives, grouting materials.

4.Adhesives: Adhesives for metals, glass, wood, and stone.

5.Fiber winding, fiberglass reinforced resin.

6.Stabilizers for polyvinyl chloride (PVC)

 

▲Schematic Representation of the Typical Structure of Bisphenol A Epoxy Resin

 

Bisphenol F Epoxy

 

Bisphenol F epoxy resin is synthesized from phenol and formaldehyde in an acidic environment, followed by condensation with epichlorohydrin. Structurally similar to bisphenol A epoxy, but lacks two methyl groups, bisphenol F exhibits lower viscosity, typically half or even lower compared to bisphenol A epoxy. This lower viscosity makes it more manageable and suitable for solvent-free coatings or casting materials, such as laminates, carbon fiber composites, etc. Blending bisphenol A and bisphenol F epoxies is a common practice to maintain low viscosity while achieving heat resistance and good electrical properties simultaneously. Its applications include:

 

1.Coatings: Ambient cure coatings, varnishes, anti-corrosive coatings, marine coatings, industrial coatings.

2.Electrical and electronic fields: Casting, impregnation, encapsulation, lamination, and insulation applications.

3.Civil engineering and construction: Cementitious concrete structures, waterproofing, anchoring adhesives, sealants, and grouting materials.

4.Adhesives: Adhesives for metals, glass, wood, stone.

5.Fiber winding, fiberglass, carbon fiber, fabric-reinforced resins.

 

▲Schematic Representation of the Typical Structure of Bisphenol F Epoxy Resin

 

Brominated Epoxy

 

Due to its excellent heat resistance, flame retardancy, chemical resistance, and electrical properties, it is commonly used in flame retardant-related applications. For example, in laminates, during the manufacturing process of PCBs, temperatures may exceed 200°C. Therefore, the addition of halogenated high-heat-resistant epoxy resins has significant advantages. In addition to laminates, applications in transportation vehicles such as cars and aircraft, where high specifications and safety factors are considered, materials with flame retardant advantages can meet the requirements. The main applications are listed below, including flame-retardant durable laminates in EMC or FCCL laminates, molding materials, adhesives, and encapsulants. As well as flame retardants for ENPLA.

 

▲Schematic Representation of the Typical Structure of Brominated Epoxy Resin

 

Phenolic Novolac Epoxy

 

Phenolic epoxy is synthesized from the reaction between epichlorohydrin and phenolic resin, featuring excellent chemical resistance, stability, and strength. However, compared to other materials, it exhibits relatively poor flexibility and is prone to fracture. Nevertheless, when its advantages are properly utilized, it finds applications in structural adhesives, laminates, composite materials (such as yachts, pipelines, automotive components), PCB solder resist ink (PSR Ink), and more.

 

▲Schematic Representation of the Structure of Phenolic Epoxy Resin

 

o-Cresol Novolac Epoxy, CNE

 

This type of resin, synthesized from ortho-methylphenol and epichlorohydrin, features higher water resistance and lower viscosity compared to standard phenolic epoxy due to the additional methyl side chain. Additionally, its high heat resistance and chemical resistance make it suitable for applications requiring high Tg and low CTE material properties.

 

Its applications include powder coatings, PCB inks, and other applications requiring heat resistance, chemical resistance, and waterproofing. Moreover, it also demonstrates advantages in applications requiring good electrical properties, such as encapsulation materials (EMC), laminates, and underfill for electronic components.

 

▲Schematic Representation of the Structure of Ortho-Methylphenol Phenolic Epoxy Resin

 

2.Specialty epoxy resins

 

Tri-Functional Epoxy

 

Trimethylolpropanetriglycidylether, TMPTE

 

The special application epoxy resin is primarily a tri-functional active diluent, characterized by its significantly low viscosity. It is utilized to dilute and reduce the viscosity of resins. It has wide applicability, being compatible with most epoxy resins, and provides improved cross-linking. For example, mixing 20% of this diluent with 80% standard bisphenol A epoxy resin can be applied in applications requiring high adhesion capability.

 

▲Schematic representation of Trimethylolpropane Triacrylate

 

Silicone-modified trifunctional epoxy resin

 

The epoxy resin, modified with silicone, produces a highly adhesive material that significantly enhances bonding capabilities between dissimilar materials, such as organic and inorganic substrates, as well as glass and non-ferrous metals. It finds applications in coatings, paints, electronics, electrical engineering, fiberglass reinforcement, carbon fiber reinforcement, and more. The comparison between silicone-modified epoxy and standard bisphenol A epoxy below demonstrates excellent adhesion and shear strength on aluminum and glass substrates.

 

Component		Silicone-modified trifunctional epoxy resin	Standard bisphenol A epoxy resin
Coating adhesion test (% residual)	Iron substrate	100	100
	Aluminum substrate	100	37
	Glass substrate	100	20
Shear adhesive strength (Kgf/cm2)		80.8	34.1

 

▲Schematic Representation of the Structure of Silicone-Modified Trifunctional Epoxy Resin

 

Tera-Functional Epoxy

 

a.Tetraglycidyl 4,4′-diaminodiphenyl-methane, TGDDM

 

This type of resin is currently the most widely used tetrafunctional epoxy in applications. With four amino groups, it provides greater rigidity and excellent heat resistance, chemical resistance, and radiation resistance. Due to its high Tg characteristics (typically exceeding 230°C), it is commonly used as a material for aerospace industries, encapsulation, or applications requiring high-temperature adhesion.

 

▲Structure of TGDDM

 

b.Glyoxal, oligomeric reaction products with 1-chloro-2,3-epoxypropane and phenol

 

Nitrogen-free tetrafunctional epoxy resin has recently garnered significant attention. This type is one of them. Although TGDDM is widely used, its nitrogen atoms lead to relatively high water absorption, reduced thermal stability, and closer epoxy group distances. To address these issues, this type of tetrafunctional epoxy resin avoids the use of nitrogen atoms and evenly distributes the epoxy groups as much as possible, which contributes to the integrity of the overall reaction.

 

▲Schematic representation of the structure of the oligomer produced from formaldehyde, 1-chloro-2,3-epoxypropane, and phenol

 

Cycloaliphatic Epoxy

 

Cycloaliphatic epoxy resin is a type of resin containing two double bonds in the cycloaliphatic group, obtained through oxidation reaction using peroxyacetic acid or organic peroxides. The two epoxy groups are attached to the cycloaliphatic ring, forming a tight structure after the reaction, which can achieve a lower coefficient of thermal expansion (Low CTE) at certain temperatures. Additionally, due to the absence of benzene rings, it exhibits excellent weather resistance and UV resistance, making it suitable for applications such as weather-resistant coatings, electronic insulation, potting, etc. Moreover, cycloaliphatic epoxy resin also possesses advantages such as transparency and low viscosity, making it suitable for solvent-free applications.

 

▲Schematic Representation of Standard Cycloaliphatic Epoxy Resin Structure

 

▲Schematic representation of long-chain cycloaliphatic epoxy resin structure

 

Dicyclopentdiene, DCPD

 

Derived from the polyaddition compound of dicyclopentadiene and 2,6-dimethylphenol. It possesses excellent low moisture absorption, low dielectric constant, considerable adhesion, as well as good heat and chemical resistance. Suitable for high-performance epoxy molding compounds (EMC) in semiconductor and laminate applications. The major advantage of using this product in semiconductor packaging is its significantly lower moisture absorption compared to previous generations of epoxy resins, such as cresol novolac epoxy (CNE) or biphenyl epoxy resins. In addition to molding materials, it can also be applied in electronic encapsulation materials, flexible materials, solder resist inks, and other fields.

 

▲Schematic representation of alkylphenol-based DCPD epoxy resin

 

Core Shell Rubber Modified Epoxy

 

Designed for modification of epoxy resins with methyl methacrylate-butadiene-styrene (MBS) core-shell rubber. The design ratio of CSR is typically between 25-40%, exhibiting excellent flexibility, bonding strength, peel strength, impact resistance, fracture toughness, elastic modulus, shear, and tensile strength. Applications include reinforcement sealing for automotive body panels (BPR sealer), hem-flange adhesive, structural adhesives, aerospace industry, and adhesive for flooring and tiles.

 

▲Core-shell structure illustration in toughened epoxy resin

 

Further Reading:（Toughened epoxy resin-CSR core-shell technology to solves the problem of brittleness!! 

 

Summary of Epoxy Resin Properties and Applications

 

Types of Epoxy Resins	Physical parameters			Characteristics*					Applications
	Viscosity (cps)	Free chlorine (ppm)	Total chlorine (ppm)	Weather resistance	Heat resistance	Chemical resistance	Low moisture absorption	Tg
Low chlorine bisphenol A type	3000-5000	<50	<200	-	++	++	-	-	Chlorine content adjustment for formulation purposes
Low chlorine bisphenol F type	1000-2000	<200	<700	-	++	++	-	-	Chlorine content adjustment for formulation purposes, viscosity adjustment purposes
Brominated epoxy	Solid	<500	-	-	++	-	-	-	Flame retardant formulation adjustment
Phenolic varnish	3000-4500	<350	-	-	++	++	+	++	Chemical resistance formulation adjustment
Ortho-cresol phenolic	10000-20000	<1000	-	-	+++	+++	+	++	Chemical resistance formulation adjustment
Trifunctional trimethylolpropane-based	4000-6000	<800	-	-	+	+	-	-	Formulation adjustment for improved adhesion and reduced viscosity, suitable for use as a diluent
Tri-functional silicon-modified	9000-150000	<500	-	-	++	++	-	-	Formulation adjustment for improved adhesion
Tetra-functional TGDDM	Solid	-	-	-	++	++	-	+++	Formulation adjustment for improved adhesion
Tetra-functional Glyoxal-Based	solid	<3000	-	-	++	++	++	+++	Formulation adjustment for improved adhesion
Cycloaliphatic-based	200-400	-	-	++	++	++	++	-	Can be applied in solvent-free formulations, adjusting viscosity, and can be used as a diluent
Bicyclopentadiene-based	solid	<200	<1000	++	++	++	+++	++	High heat resistance application, sealant material
CSR Toughened	20000-200000	-	-	-	++	++	-	-	Flexibility adjustment to improve brittleness

*+ The more, the better the effectiveness of this characteristic

 

Classification of Epoxy Curing Agents

 

Products formed by epoxy typically result from the reaction of two components, A and B. Component A usually represents the epoxy resin, while component B represents the curing agent. Since the curing agent also accounts for a significant portion of the formulation, its role is crucial in determining properties such as chemical resistance, heat resistance, and mechanical strength of the final product. This underscores the importance of curing agents in influencing the ultimate properties. Below are several common types of curing agents along with their characteristics and suitable applications.

 

Types	Curing agents	Reaction temperature	Temperature type	Heat resistance	Chemical resistance	Low moisture absorption	Curing speed	Feature
Dicyandiamide	Dicyandiamide, DICY	120-150 degree	Medium temperature	++	++	+	-	The reaction temperature alone is high, and it is usually used together with accelerators such as imidazole to reduce the reaction temperature. It is commonly used in latent curing agents.
Fatty amine	Diethylenetriamine, DETA Triethylene Tetramine, TETA	80-120 degree	Room temperature, moderate to low temperature	++	++	+	++	Common epoxy curing agents, excellent heat resistance, chemical resistance. Suitable for applications such as casting, coatings, wire and cable varnishes, etc.
Imidazole type	Imidazole 2-Methylimidazole, 2-MI 2-phenyl Imidazole, 2-PZ	80-120 degree	Medium temperature	+	+	+	+++	Fast reaction speed
Anhydride	Hexahydro-4-methylphthalic anhydride, 4MHHPA Tetra Hydro Phthalic Anhydride, THPA	>150 degree	High temperature	++	++	-	++	The curing temperature is high, but it offers excellent electrical and heat resistance, while its moisture resistance is relatively poor.
Aromatic amine	Diamino Diphenyl Sulfone, DDS Diaminodiphenylmethane, DDM 4,4'-Diaminodicyclohexyl methane, PACM	> 150 degree	High temperature	++	++	++	-	Characterized by excellent chemical resistance, heat resistance, low water absorption, and high glass transition temperature.
Phenolic	Novolac resin	>150 degree	High temperature	++	++	+	-	Compared to other curing agents, this type exhibits excellent heat and chemical resistance, making it suitable for powder coating applications.

 

Examples of epoxy resin and curing agent combinations

 

Molding Compound

 

Component		Values
Formulation (phr)	Standard Solid Epoxy Resin	100
	DDM	9.6
	Calcium Stearate	3
	Silicon Dioxide	250
Reaction conditions	Blend (oC/min)	25-57/5
	Aging (min at 80oC)	43
Physical properties	H.D.T(oC)	105.5
	Barcol hardness	52
	Water absorption (1hr Boiling, wt.%)	0.212
	Flexural strength (KG/mm2)	12.7
	Volume resistivity (ohm-cm)	1.7x1014

*Potting condition: Pressure 100-200 kg/cm² at 150°C for 10 minutes

 

Paint Application Reference Formulation Ingredients and Physical Properties

 

Component			Values
Formulation Ingredients (wt.%)	Phenolic varnish epoxy resin		32
	Titanium dioxide		18
	Talc		20
	Diluent		11
	Additive		3
	Epoxy curing agent		16
Reaction Conditions	Mixing ratio of A:B (wt.%)		84:16
	Volume concentration of dye (%)		30
	Volume solid content (%)		65
	Curing conditions		25oC for 7 days
Physical Properties	Application time		4 hr 30 min
	Drying time	Tack-free drying	2 hr
		Full drying	7 hr 30 m
	Shore hardness		77
	Abrasion resistance (mg/weight loss)		61
	Chemical resistance (25oC for 14 days)	50% NaOH	No foaming, softening, swelling, loss of adhesion
		Phenol
		Tetrahydrofuran
		Methanol

 

Reference formulation ingredients and physical properties of potting materials

 

Component			Values
Formula (wt.%)	Ortho-cresol novolac epoxy resin Silica micro powder Phenol-formaldehyde resin TPP Toughening agent Other additives total		16 70 8.5 0.3 2 3.2 100
Physical properties	Vortex flow length (inch)		33.5
	Gel time (sec)		26.9
	Resin exudation (mm)		1.43
	Hardness		88
	Flexural strength (Kgf/mm2)		14.5
	Flexural modulus (Kgf/mm2)		1,388
	Purity	Cl- (ppm)	4.5
		Fe+ (ppm)	2.2
		Na+(ppm)	3.3

 

Reference formulation ingredients and physical properties applied to laminated panels

 

Component		Ratio
Formula	Ortho-cresol novolac epoxy resin MEK DDS BF3MEA BDMA Diluent	100 25 - 63.6 1.1 -
Reaction conditions	B-Stage(oC/min)	130/10
	Press(Kg/cm2/oC/min)	15/170/120
Physical properties	Barcol hardness	70
	Flexural strength(Kg/mm2)	54.2
	Tensile strength (Kg/mm2)	39.5
	Volume resistivity (ohm-cm)	1.59x1016
	Resin content (%)	36.7